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    <element id="VSTA_ChangeInPutAndNoteTermExtensionOptionAndWarrantLiabilities" name="ChangeInPutAndNoteTermExtensionOptionAndWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccountsPayableSettledByIssuanceOfStockOrNotesPayableAndStock" name="AccountsPayableSettledByIssuanceOfStockOrNotesPayableAndStock" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InsurancePremiumsSettledByIssuingNotePayable" name="InsurancePremiumsSettledByIssuingNotePayable" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RecognitionOfWarrantLiabilityUponIssuanceToPlatinumOfOctober2012" name="RecognitionOfWarrantLiabilityUponIssuanceToPlatinumOfOctober2012" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProceedsFromSaleOfCommonStockForCashIncludingExercisesofWarrantsUnderDiscountWarrantExerciseProgramShares" name="ProceedsFromSaleOfCommonStockForCashIncludingExercisesofWarrantsUnderDiscountWarrantExerciseProgramShares" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProceedsFromSaleOfCommonStockForCashIncludingExercisesOfWarrantsUnderDiscountWarrantExerciseProgram" name="ProceedsFromSaleOfCommonStockForCashIncludingExercisesOfWarrantsUnderDiscountWarrantExerciseProgram" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PaymentsOnNoteReceivableForSaleOfStock" name="PaymentsOnNoteReceivableForSaleOfStock" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_CurrentPortionOfNotesPayableToRelatedPartiesNetOfDiscountOf54500AtMarch312015AndAccruedInterest" name="CurrentPortionOfNotesPayableToRelatedPartiesNetOfDiscountOf54500AtMarch312015AndAccruedInterest" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_NotesPayableDiscountRelatedParties" name="NotesPayableDiscountRelatedParties" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_WarrantLiabilityPolicyTextBlock" name="WarrantLiabilityPolicyTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrepaidExpensesTableTextBlock" name="PrepaidExpensesTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ScheduleOfSharesReservedForFutureIssuance" name="ScheduleOfSharesReservedForFutureIssuance" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_ScheduleDescriptionOfUnits" name="ScheduleDescriptionOfUnits" 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" />
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    <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" />
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    <element id="VSTA_SeniorNote5Member" name="SeniorNote5Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_MandFNoteAMember" name="MandFNoteAMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_BurrPilgerMayerMember" name="BurrPilgerMayerMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DesjardinsMember" name="DesjardinsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_McCarthyTetraultMember" name="McCarthyTetraultMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_MayTwoThousandElevenMorrisonFoersterMember" name="MayTwoThousandElevenMorrisonFoersterMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_ServiceProvidersCurrentMember" name="ServiceProvidersCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_InsuranceCompanyMember" name="InsuranceCompanyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_VendorMember" name="VendorMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_VendorCurrentMember" name="VendorCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_MFNoteAMember" name="MFNoteAMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_Warrant2Member" name="Warrant2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant3Member" name="Warrant3Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant4Member" name="Warrant4Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant5Member" name="Warrant5Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant6Member" name="Warrant6Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant7Member" name="Warrant7Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_USNIHMember" name="USNIHMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UHNMember" name="UHNMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_DenominatorAbstract" name="DenominatorAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_FairValueMeasurementsDetailsAbstract" name="FairValueMeasurementsDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueMeasurementsDetails1Abstract" name="FairValueMeasurementsDetails1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrepaidExpensesDetailsAbstract" name="PrepaidExpensesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_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" />
    <element id="VSTA_WarrantsOutstanding" name="WarrantsOutstanding" abstract="true" nillable="true" xbrli:periodType="duration" 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_PursuantToWarrantsToPurchaseCommonStock" name="PursuantToWarrantsToPurchaseCommonStock" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_IncomeTaxesDetailsAbstract" name="IncomeTaxesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_LicensingAndCollaborativeAgreementsDetailsTextualsAbstract" name="LicensingAndCollaborativeAgreementsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PeriodicResearchAndDevelopmentArrangementWithFederalGovernmentCustomerFundingToOffsetCostsIncurred" name="PeriodicResearchAndDevelopmentArrangementWithFederalGovernmentCustomerFundingToOffsetCostsIncurred" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" 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_WeightedAverageExercisePrice" name="WeightedAverageExercisePrice" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_StockOptionPlansAnd401KPlanDetails3Abstract" name="StockOptionPlansAnd401KPlanDetails3Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_StockOptionPlansAnd401KPlanDetailsNarrativeAbstract" name="StockOptionPlansAnd401KPlanDetailsNarrativeAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationAxis" name="ShareBasedCompensationAxis" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" />
    <element id="VSTA_ShareBasedCompensationDomain" name="ShareBasedCompensationDomain" abstract="true" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" 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_CommitmentsContingenciesGuaranteesAndIndemnificationsDetailsAbstract" name="CommitmentsContingenciesGuaranteesAndIndemnificationsDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_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_CommitmentsContingenciesGuaranteesAndIndemnificationsDetails3Abstract" name="CommitmentsContingenciesGuaranteesAndIndemnificationsDetails3Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SubsequentEventsDetailsTextualsAbstract" name="SubsequentEventsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SupplementalFinancialInformationDetails2Abstract" name="SupplementalFinancialInformationDetails2Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
</schema>
