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	  <link:definition>0027 - Disclosure - Fair Value Measurements (Tables)</link:definition>
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	  <link:definition>0030 - Disclosure - Convertible Promissory Notes and Other Notes Payable (Tables)</link:definition>
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	  <link:definition>0039 - Disclosure - Summary of Significant Accounting Policies (Details)</link:definition>
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	  <link:definition>0047 - Disclosure - 7. Convertible Promissory Notes and Other Notes Payable (Details 1)</link:definition>
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	  <link:definition>0048 - Disclosure - 7. Convertible Promissory Notes and Other Notes Payable (Details Textuals)</link:definition>
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	  <link:definition>0049 - Disclosure - 7. Convertible Promissory Notes and Other Notes Payable (Details Textuals 2)</link:definition>
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	  <link:definition>0050 - Disclosure - 9. Capital Stock (Details)</link:definition>
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	  <link:definition>0051 - Disclosure - 9. Capital Stock (Details 1)</link:definition>
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	  <link:definition>0052 - Disclosure - Capital Stock (Details 2)</link:definition>
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	  <link:definition>0053 - Disclosure - 9. Capital Stock (Details Textuals)</link:definition>
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	  <link:definition>0054 - Disclosure - Research and Development Expenses (Details Narrative)</link:definition>
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	  <link:definition>0055 - Disclosure - Income Taxes (Details)</link:definition>
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	  <link:definition>0056 - Disclosure - Income Taxes (Details 1)</link:definition>
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    <element id="VSTA_CommonStockExchangedForSeriesPreferredUnderAgreementsWithPlatinumNoteAndWarrantExchangeAgreementShares" name="CommonStockExchangedForSeriesPreferredUnderAgreementsWithPlatinumNoteAndWarrantExchangeAgreementShares" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantsIssuedUponExerciseOfModifiedWarrants" name="FairValueOfWarrantsIssuedUponExerciseOfModifiedWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfSharesIssuedInSettlementOfAccountsPayable" name="FairValueOfSharesIssuedInSettlementOfAccountsPayable" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfSharesIssuedInSettlementOfAccountsPayableShares" name="FairValueOfSharesIssuedInSettlementOfAccountsPayableShares" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ModificationOfNoteReceivableFromSaleOfStock" name="ModificationOfNoteReceivableFromSaleOfStock" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantIssuedToCatoHoldingCompanyInConnectionWithNotePayableRestructure" name="FairValueOfWarrantIssuedToCatoHoldingCompanyInConnectionWithNotePayableRestructure" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantIssuedInConnectionWithAccountsPayableRestructure" name="FairValueOfWarrantIssuedInConnectionWithAccountsPayableRestructure" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantIssued2InConnectionWithAccountsPayableRestructure" name="FairValueOfWarrantIssued2InConnectionWithAccountsPayableRestructure" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantsIssuedInConnectionWithAccruedInterestOnUnderlyingNotes" name="FairValueOfWarrantsIssuedInConnectionWithAccruedInterestOnUnderlyingNotes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SaleOfUnitsInWinter2012PrivatePlacementNetAmount" name="SaleOfUnitsInWinter2012PrivatePlacementNetAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SaleOfUnitsInWinter2012PrivatePlacementNetShares" name="SaleOfUnitsInWinter2012PrivatePlacementNetShares" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExchangeOfFebruary2012ConvertibleNotesAmount" name="ExchangeOfFebruary2012ConvertibleNotesAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExchangeOfFebruary2012ConvertibleNotesShares" name="ExchangeOfFebruary2012ConvertibleNotesShares" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantsIssuedToBankerInConnectionWithExchangeOfFebruary2012ConvertibleNotes" name="FairValueOfWarrantsIssuedToBankerInConnectionWithExchangeOfFebruary2012ConvertibleNotes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PremiumOfFairValueOverFaceValueOfExchangeNoteIssuedToPlatinum" name="PremiumOfFairValueOverFaceValueOfExchangeNoteIssuedToPlatinum" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProceedsAllocatedToBeneficialConversionFeatureOfInvestmentNotesIssuedToPlatinumInOctober2012February2013AndMarch2013" name="ProceedsAllocatedToBeneficialConversionFeatureOfInvestmentNotesIssuedToPlatinumInOctober2012February2013AndMarch2013" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IncrementalFairValueOfWarrantModificationsInFeburary2013" name="IncrementalFairValueOfWarrantModificationsInFeburary2013" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesToFinancialStatementsAbstract" name="NotesToFinancialStatementsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_HistoryAndOrganizationTextBlock" name="HistoryAndOrganizationTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedExpensesTextBlock" name="AccruedExpensesTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockTextBlock" name="CapitalStockTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_LicensingAndCollaborativeAgreementsTextBlock" name="LicensingAndCollaborativeAgreementsTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SummaryOfSignificantAccountingPoliciesPoliciesAbstract" name="SummaryOfSignificantAccountingPoliciesPoliciesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ChangeInAuthorizedNumberOfSharesPolicy" name="ChangeInAuthorizedNumberOfSharesPolicy" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RecentlyAdoptedAccountingStandardsPolicyTextBlock" name="RecentlyAdoptedAccountingStandardsPolicyTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SummaryOfSignificantAccountingPoliciesTablesAbstract" name="SummaryOfSignificantAccountingPoliciesTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FiarValueMeasurementsTablesAbstract" name="FiarValueMeasurementsTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ChangesInLevel3Liabilities" name="ChangesInLevel3Liabilities" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedExpensesTablesAbstract" name="AccruedExpensesTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableTablesAbstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableTableTextBlock" name="ConvertiblePromissoryNotesAndOtherNotesPayableTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExchangeWarrantAndInvestmentWarrantsTableTextBlock" name="ExchangeWarrantAndInvestmentWarrantsTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NoteDiscountAndEffectiveInterestRateTableTextBlock" name="NoteDiscountAndEffectiveInterestRateTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IncrementalValueOfAmendedMfWarrant" name="IncrementalValueOfAmendedMfWarrant" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockTablesAbstract" name="CapitalStockTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BlackscholesOptionPricingModelTableTextBlock" name="BlackscholesOptionPricingModelTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BlackscholesOptionPricingModelModificationsTableTextBlock" name="BlackscholesOptionPricingModelModificationsTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BlackscholesOptionPricingModelOnAssumptionsTableTextBlock" name="BlackscholesOptionPricingModelOnAssumptionsTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OtherWarrantModificationsAssumptionsTableTextBlock" name="OtherWarrantModificationsAssumptionsTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BlackscholesOtherWarrantModificationsOptionPricingModelOnAssumptionsTableTextBlock" name="BlackscholesOtherWarrantModificationsOptionPricingModelOnAssumptionsTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ReservedSharesTableTextBlock" name="ReservedSharesTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ScheduleofSharebasedCompensationExpenseTextBlock" name="ScheduleofSharebasedCompensationExpenseTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BlackscholesOptionPricingModelTableAssumptionsTextBlock" name="BlackscholesOptionPricingModelTableAssumptionsTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_StockOptionActivitySummaryTextBlock" name="StockOptionActivitySummaryTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_StockOptionsOutstandingAndExercisableUnderCompanysOptionPlanTextBlock" name="StockOptionsOutstandingAndExercisableUnderCompanysOptionPlanTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ScheduleOfFutureMinimumLeasePaymentsForLongTermDebtTableTextBlock" name="ScheduleOfFutureMinimumLeasePaymentsForLongTermDebtTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SubsequentEventsTablesAbstract" name="SubsequentEventsTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_QuarterlyFinancialInformationTableTextBlock" name="QuarterlyFinancialInformationTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_HistoryAndOrganizationDetailsTextualsAbstract" name="HistoryAndOrganizationDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AwardedByU.S.NationalInstitutes" name="AwardedByU.S.NationalInstitutes" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OutstandingShares" name="OutstandingShares" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareRepurchase" name="ShareRepurchase" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SharesOutstandingAtMerger" name="SharesOutstandingAtMerger" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OutstandingShareRepresentation" name="OutstandingShareRepresentation" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NumberOfSharesAuthorizedFollowingAmendment" name="NumberOfSharesAuthorizedFollowingAmendment" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NumberOfPreferredSharesAuthorizedFollowingAmendment" name="NumberOfPreferredSharesAuthorizedFollowingAmendment" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeriesPreferredStockShares" name="SeriesPreferredStockShares" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeriesPreferredShareParValue" name="SeriesPreferredShareParValue" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConversionRateSeriesPreferredShares" name="ConversionRateSeriesPreferredShares" nillable="true" xbrli:periodType="instant" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConversionTotalSeriesPreferredToCommonStock" name="ConversionTotalSeriesPreferredToCommonStock" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConversionToWarrantWarrantValue" name="ConversionToWarrantWarrantValue" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantExercisePrice" name="WarrantExercisePrice" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BasisOfPresentationAndGoingConcernDetailsTextualsAbstract" name="BasisOfPresentationAndGoingConcernDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpectedCashExpenditure" name="ExpectedCashExpenditure" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProceedsFromIssuancesOfPrivatePlacement" name="ProceedsFromIssuancesOfPrivatePlacement" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SummaryOfSignificantAccountingPoliciesDetailsAbstract" name="SummaryOfSignificantAccountingPoliciesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantSharesIssuableUponExerciseNotePurchaseAndExchangeAgreement" name="WarrantSharesIssuableUponExerciseNotePurchaseAndExchangeAgreement" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OutstandingOptionsUnder2008And1999StockIncentivePlanAnd1998ScientificAdvisoryBoardPlan" name="OutstandingOptionsUnder2008And1999StockIncentivePlanAnd1998ScientificAdvisoryBoardPlan" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OutstandingWarrantsToPurchaseCommonStock" name="OutstandingWarrantsToPurchaseCommonStock" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_February201212ConvertiblePromissoryNotesAndAccruedInterest1" name="February201212ConvertiblePromissoryNotesAndAccruedInterest1" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Total" name="Total" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueMeasurementsDetailsAbstract" name="FairValueMeasurementsDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue1" name="FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue1" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueMeasurementsDetailsTextualsAbstract" name="FairValueMeasurementsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OriginalPrincipleBalanceOfNotes" name="OriginalPrincipleBalanceOfNotes" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RepaymentOfDebtInPercentage" name="RepaymentOfDebtInPercentage" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IssuedWarrantToPurchaseCommonStock" name="IssuedWarrantToPurchaseCommonStock" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ChangeInFairValueOfTheWarrantLiability" name="ChangeInFairValueOfTheWarrantLiability" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IncreaseInPutOptionAndNoteTermExtensionOptionLiabilities" name="IncreaseInPutOptionAndNoteTermExtensionOptionLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RemainingPutOptionAndNoteTermExtensionOptionLiabilities" name="RemainingPutOptionAndNoteTermExtensionOptionLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ComputersAndNetworkEquipment" name="ComputersAndNetworkEquipment" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OfficeFurnitureAndEquipment" name="OfficeFurnitureAndEquipment" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Av101AcquisitionDetailsTextualsAbstract" name="Av101AcquisitionDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeriesB1SharesIssued" name="SeriesB1SharesIssued" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeriesB1ValuePerShare" name="SeriesB1ValuePerShare" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PremergerPurchasePrice" name="PremergerPurchasePrice" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AggregateSupportFunding" name="AggregateSupportFunding" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedExpensesDetailsAbstract" name="AccruedExpensesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedResearchAndDevelopmentExpenses" name="AccruedResearchAndDevelopmentExpenses" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedPlacementAgentFees" name="AccruedPlacementAgentFees" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedRegistrationRightsPayments" name="AccruedRegistrationRightsPayments" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableDetailsAbstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableDetails2Abstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetails2Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_ExercisePriceInvestmentWarrants" name="ExercisePriceInvestmentWarrants" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExchangeWarrantNumberOfShares" name="ExchangeWarrantNumberOfShares" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueAtDateOfIssuance" name="FairValueAtDateOfIssuance" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ComponentsOfDiscountAndEffectiveInterestRateForInvestmentNotes" name="ComponentsOfDiscountAndEffectiveInterestRateForInvestmentNotes" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DiscountAttributableToFairValueOfWarrant" name="DiscountAttributableToFairValueOfWarrant" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DiscountAttributableToBeneficialConversionFeature" name="DiscountAttributableToBeneficialConversionFeature" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InceptionDateCarryingValue" name="InceptionDateCarryingValue" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_MarketPricePerShare" name="MarketPricePerShare" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueAssumptionsExercisePrice1" name="FairValueAssumptionsExercisePrice1" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValuePerShare" name="FairValuePerShare" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableDetailsTextuals1Abstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetailsTextuals1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_July2012PlatinumNoteAmount" name="July2012PlatinumNoteAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_August2012PlatinumNote" name="August2012PlatinumNote" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_JulyAndAugustNoteRate" name="JulyAndAugustNoteRate" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpectedGrossNoteProceeds" name="ExpectedGrossNoteProceeds" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_February2012Note" name="February2012Note" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FebruaryNoteRate" name="FebruaryNoteRate" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantValue" name="WarrantValue" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AggregateShareValue" name="AggregateShareValue" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RegistrationRightsAgreementLiability" name="RegistrationRightsAgreementLiability" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_October2012PlatinumNotePrincipalAmount" name="October2012PlatinumNotePrincipalAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_October2012PlatinumNoteTranchAmount" name="October2012PlatinumNoteTranchAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_October2012PlatinumNoteGrossProceeds" name="October2012PlatinumNoteGrossProceeds" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrivatePlacements" name="PrivatePlacements" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrivatePlacementUnitPrice" name="PrivatePlacementUnitPrice" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FinancingRequiredToSellNote" name="FinancingRequiredToSellNote" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableDetailsTextuals3Abstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetailsTextuals3Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockDetailsAbstract" name="CapitalStockDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExercisePricePerShareWeightedAvg" name="ExercisePricePerShareWeightedAvg" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpectedTermYearsMaximum" name="ExpectedTermYearsMaximum" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WeightedAverageFairValuePerShare" name="WeightedAverageFairValuePerShare" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockDetails1Abstract" name="CapitalStockDetails1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantsOptionAbstract" name="WarrantsOptionAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExercisePrice" name="ExercisePrice" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpirationDate" name="ExpirationDate" nillable="true" xbrli:periodType="instant" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockDetails2Abstract" name="CapitalStockDetails2Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantSharesIssuableToPlatinumUponExerciseOfCommonStockWarrantUponExchangeOfSeriesPreferredStockUnderTermsOfOctober112012NotePurchaseAndExchangeAgreement" name="WarrantSharesIssuableToPlatinumUponExerciseOfCommonStockWarrantUponExchangeOfSeriesPreferredStockUnderTermsOfOctober112012NotePurchaseAndExchangeAgreement" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OfSharesIssuableUponConversionOf10ConvertibleExchangeNoteAndInvestmentNotesIssuedToPlatinumInOctober2012February2013AndMarch2013IncludingInterestAccruedThroughMaturity2" name="OfSharesIssuableUponConversionOf10ConvertibleExchangeNoteAndInvestmentNotesIssuedToPlatinumInOctober2012February2013AndMarch2013IncludingInterestAccruedThroughMaturity2" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PursuantToWarrantsToPurchaseCommonStock" name="PursuantToWarrantsToPurchaseCommonStock" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IssuablePursuantToAccruedInterestThroughMaturityOnOutstandingPromissoryNotesIssuedToMorrisonFoersterCatoResearchLtd.AndUniversityHealthNetwork" name="IssuablePursuantToAccruedInterestThroughMaturityOnOutstandingPromissoryNotesIssuedToMorrisonFoersterCatoResearchLtd.AndUniversityHealthNetwork" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PursuantToWarrantsToPurchaseCommonStockTotal" name="PursuantToWarrantsToPurchaseCommonStockTotal" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PursuantToStockIncentivePlans" name="PursuantToStockIncentivePlans" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SubjectToOutstandingOptionsUnder2008And1999StockIncentivePlans" name="SubjectToOutstandingOptionsUnder2008And1999StockIncentivePlans" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UponSalesOfAdditionalUnitsPursuantTo2012PrivatePlacementOfUnits" name="UponSalesOfAdditionalUnitsPursuantTo2012PrivatePlacementOfUnits" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockDetailsTextualsAbstract" name="CapitalStockDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SharePrice1" name="SharePrice1" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerShareWeightedAveragePriceOfSharesPurchased1" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardPerShareWeightedAveragePriceOfSharesPurchased1" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ContractualTerm1" name="ContractualTerm1" nillable="true" xbrli:periodType="duration" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant1" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant1" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CommonStockValue1" name="CommonStockValue1" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AmountsDueForServices" name="AmountsDueForServices" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CommonStockPurchseDiscription" name="CommonStockPurchseDiscription" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IncreasedValueOfWarrantsExercised" name="IncreasedValueOfWarrantsExercised" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CommonStockOwned" name="CommonStockOwned" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SecuredConvertiblePromissoryNote1" name="SecuredConvertiblePromissoryNote1" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FinancingTransactionsGrossProceeds" name="FinancingTransactionsGrossProceeds" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InvestmentInQualifiedFinancing" name="InvestmentInQualifiedFinancing" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AmountOfCommitmentsFromOtherInvestors" name="AmountOfCommitmentsFromOtherInvestors" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PeriodOfCommitmentsFromOtherInvestors2" name="PeriodOfCommitmentsFromOtherInvestors2" nillable="true" xbrli:periodType="instant" type="xbrli:durationItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IncomeTaxesDetailsAbstract" name="IncomeTaxesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_LossesNotBenefitted" name="LossesNotBenefitted" nillable="true" xbrli:periodType="duration" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_EffectiveIncomeTaxRateReconciliationOther" name="EffectiveIncomeTaxRateReconciliationOther" nillable="true" xbrli:periodType="duration" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_IncomeTaxesDetails1Abstract" name="IncomeTaxesDetails1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BasisDifferencesInFixedAssets" name="BasisDifferencesInFixedAssets" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpectedDividendYield" name="ExpectedDividendYield" nillable="true" xbrli:periodType="duration" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueAssumptionsRiskFreeInterestRates" name="FairValueAssumptionsRiskFreeInterestRates" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpectedYearTerm" name="ExpectedYearTerm" nillable="true" xbrli:periodType="instant" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardVolatilityRate" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardVolatilityRate" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValuePerShareAtGrantDate" name="FairValuePerShareAtGrantDate" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NumberOfShares" name="NumberOfShares" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod2" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOtherIncreasesDecreasesInPeriod2" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WeightedAverageExercisePrice" name="WeightedAverageExercisePrice" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingPeriodIncreaseDecreaseWeightedAverageExercisePrice2" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingPeriodIncreaseDecreaseWeightedAverageExercisePrice2" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedPartyTransactionsDetailsTextualsAbstract" name="RelatedPartyTransactionsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CrlNoteIntrestExpense" name="CrlNoteIntrestExpense" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CommitmentsContingenciesGuaranteesAndIndemnificationsDetailsAbstract" name="CommitmentsContingenciesGuaranteesAndIndemnificationsDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_LeasedLaboratoryAndComputerEquipmentNet" name="LeasedLaboratoryAndComputerEquipmentNet" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CommitmentsContingenciesGuaranteesAndIndemnificationsDetails1Abstract" name="CommitmentsContingenciesGuaranteesAndIndemnificationsDetails1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalLeaseObligationsEquipmentCurrent" name="CapitalLeaseObligationsEquipmentCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CommitmentsContingenciesGuaranteesAndIndemnificationsDetails2Abstract" name="CommitmentsContingenciesGuaranteesAndIndemnificationsDetails2Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FutureMinimumPaymentsPrincipal" name="FutureMinimumPaymentsPrincipal" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FutureMinimumPrincipalPayments" name="FutureMinimumPrincipalPayments" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SubsequentEventsDetailsAbstract" name="SubsequentEventsDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FutureMinimumPaymentOperatingLease" name="FutureMinimumPaymentOperatingLease" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SharesOfCommonStockRetroactivelyReflectedAsOutstanding" name="SharesOfCommonStockRetroactivelyReflectedAsOutstanding" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SupplementalFinancialInformationDetails2Abstract" name="SupplementalFinancialInformationDetails2Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
</schema>
