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    <element id="VSTA_ServiceProvidersCurrentMember" name="ServiceProvidersCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ServiceProvidersNoncurrentMember" name="ServiceProvidersNoncurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <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" />
    <element id="VSTA_UnrelatedPartiesMember" name="UnrelatedPartiesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnrelatedPartiesCurrentMember" name="UnrelatedPartiesCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnrelatedPartiesNoncurrentMember" name="UnrelatedPartiesNoncurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CatoNote1Member" name="CatoNote1Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CatoNote2Member" name="CatoNote2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedPartiesAggregateMember" name="RelatedPartiesAggregateMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedPartiesAfterDiscountMember" name="RelatedPartiesAfterDiscountMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedPartiesCurrentMember" name="RelatedPartiesCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedPartiesNoncurrentMember" name="RelatedPartiesNoncurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertibleNoteJulyMember" name="ConvertibleNoteJulyMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNoteTotalMember" name="SeniorNoteTotalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnitNotesMember" name="UnitNotesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnitNotes2Member" name="UnitNotes2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_TenpercentTotalMember" name="TenpercentTotalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_TenPercentConvertibleMember" name="TenPercentConvertibleMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_MFNoteAMember" name="MFNoteAMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_MFNoteBMember" name="MFNoteBMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnivHealthMember" name="UnivHealthMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ServiceProviderTotalMember" name="ServiceProviderTotalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ServiceNotesCurrentMember" name="ServiceNotesCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_SevenPercentNotesMember" name="SevenPercentNotesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SevenPercenCurrentMember" name="SevenPercenCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrivatePlacement2014Member" name="PrivatePlacement2014Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AmendmentWarrantsMember" name="AmendmentWarrantsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantAdjustmentMFMember" name="WarrantAdjustmentMFMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantAdjustmentCRLMember" name="WarrantAdjustmentCRLMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantAdjustmentUHNMember" name="WarrantAdjustmentUHNMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PreModificationMember" name="PreModificationMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PostModificationMember" name="PostModificationMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant1Member" name="Warrant1Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <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_Warrant8Member" name="Warrant8Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant9Member" name="Warrant9Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant10Member" name="Warrant10Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AutilionAGMember" name="AutilionAGMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrivatePlacement2013Member" name="PrivatePlacement2013Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_TechnologyLicenseMember" name="TechnologyLicenseMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConsultingAgreementforServicesMember" name="ConsultingAgreementforServicesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CBVNoteMember" name="CBVNoteMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CRLNoteMember" name="CRLNoteMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PlatinumWarrantsMember" name="PlatinumWarrantsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PlatinumStockUnitsMember" name="PlatinumStockUnitsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PlatinumNoteUnitsMember" name="PlatinumNoteUnitsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_JulyPlatinumMember" name="JulyPlatinumMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DocumentAndEntityInformationAbstract" name="DocumentAndEntityInformationAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesPayableAndAccruedInterestToRelatedParties" name="NotesPayableAndAccruedInterestToRelatedParties" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorSecuredConvertiblePromisoryNotesAndAccruedInterest" name="SeniorSecuredConvertiblePromisoryNotesAndAccruedInterest" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesPayableAndAccruedInterest" name="NotesPayableAndAccruedInterest" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesPayableAndAccruedInterestToRelatedPartiesNonCurrent" name="NotesPayableAndAccruedInterestToRelatedPartiesNonCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesReceivableFromSaleOfCommonStockToOthers" name="NotesReceivableFromSaleOfCommonStockToOthers" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndAccruedInterestNetOfDiscount" name="ConvertiblePromissoryNotesAndAccruedInterestNetOfDiscount" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorSecuredConvertibleNotesPayableDiscount" name="SeniorSecuredConvertibleNotesPayableDiscount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesPayableDiscount" name="NotesPayableDiscount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <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_PreferredStockSeriesASharesAuthorized" name="PreferredStockSeriesASharesAuthorized" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_ChangeInPutAndNoteTermExtensionOptionAndWarrantLiabilities" name="ChangeInPutAndNoteTermExtensionOptionAndWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_InterestIncomeOnNoteReceivableForStockPurchase" name="InterestIncomeOnNoteReceivableForStockPurchase" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfCommonStockGrantedForServicesFollowingMerger" name="FairValueOfCommonStockGrantedForServicesFollowingMerger" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfWarrantsGrantedForServicesFollowingMerger" name="FairValueOfWarrantsGrantedForServicesFollowingMerger" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_ProceedsFromExerciseOfModifiedWarrantsMayjune2012AndUnderDiscountedWarrantExerciseProgram2011" name="ProceedsFromExerciseOfModifiedWarrantsMayjune2012AndUnderDiscountedWarrantExerciseProgram2011" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <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_ScheduleDescriptionOfUnits" name="ScheduleDescriptionOfUnits" 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" />
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    <element id="VSTA_BasisOfPresentationAndGoingConcernDetailsTextualsAbstract" name="BasisOfPresentationAndGoingConcernDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_SharePurchaseObligationCashConsideration" name="SharePurchaseObligationCashConsideration" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_WarrantExercisePrice" name="WarrantExercisePrice" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AlternativeFinancingMinimum" name="AlternativeFinancingMinimum" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_TotalResearchAndDevelopmentExpenseRelatedToStockbasedCompensationExpense" name="TotalResearchAndDevelopmentExpenseRelatedToStockbasedCompensationExpense" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedToStockOptionGrantsIncludingExpenserelatedToModifications" name="RelatedToStockOptionGrantsIncludingExpenserelatedToModifications" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantsGrantedToOfficersAndDirectors" name="WarrantsGrantedToOfficersAndDirectors" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_TotalGeneralAndAdministrativeExpenseRelatedToStockbasedCompensationExpense" name="TotalGeneralAndAdministrativeExpenseRelatedToStockbasedCompensationExpense" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_WarrantsToPurchaseCommonStock" name="WarrantsToPurchaseCommonStock" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PotentiallyDilutiveCommonSharesAssumedConverted" name="PotentiallyDilutiveCommonSharesAssumedConverted" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DenominatorForDilutedLossPerShare" name="DenominatorForDilutedLossPerShare" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
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    <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_CostsAssociatedWithPublicOffering" name="CostsAssociatedWithPublicOffering" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_LegalFees1" name="LegalFees1" nillable="true" xbrli:periodType="instant" 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_ConvertiblePromissoryNotesAndOtherNotesPayableDetailsAbstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ReceivableWithImputedInterestDiscountOnAccruedInterest" name="ReceivableWithImputedInterestDiscountOnAccruedInterest" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ReceivableWithImputedInterestDiscountOnPrincipalAndAccruedInterest" name="ReceivableWithImputedInterestDiscountOnPrincipalAndAccruedInterest" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SecuritiesFinancingAxis" name="SecuritiesFinancingAxis" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrldt:dimensionItem" />
    <element id="VSTA_SecuritiesFinancingDomain" name="SecuritiesFinancingDomain" abstract="true" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RestrictedCommonStock" name="RestrictedCommonStock" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnitsSold" name="UnitsSold" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CashProceeds" name="CashProceeds" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_QualifiedFinancingRequirement" name="QualifiedFinancingRequirement" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AmendedWarrants" name="AmendedWarrants" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AmendedWarrantsValue" name="AmendedWarrantsValue" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RepaymentOfWarrantsValue" name="RepaymentOfWarrantsValue" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ChangeInWarrantPutAndNoteExtensionOptionAndWarrantLiabilities" name="ChangeInWarrantPutAndNoteExtensionOptionAndWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfAdditionalWarrantShares" name="FairValueOfAdditionalWarrantShares" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" 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_ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateFairValuePeriodIncreaseDecrease" name="ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateFairValuePeriodIncreaseDecrease" nillable="true" xbrli:periodType="duration" type="num:perShareItemType" 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_ExpirateDateRange" name="ExpirateDateRange" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WeightedAverageYearsToExpiration" name="WeightedAverageYearsToExpiration" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_OwnshareLendingArrangementSharesCommitment" name="OwnshareLendingArrangementSharesCommitment" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InitialFaceAmount" name="InitialFaceAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_StemCellTechnologyLicenseMaintenanceFees" name="StemCellTechnologyLicenseMaintenanceFees" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PreviousAdvancesToOfficers" name="PreviousAdvancesToOfficers" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExercisePriceAtGrantDate" name="ExercisePriceAtGrantDate" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfStock" name="FairValueOfStock" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_MonthlyPaymentForProfessionalServices" name="MonthlyPaymentForProfessionalServices" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" 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_CashAdvancesAndDeposits" name="CashAdvancesAndDeposits" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InvestmentInLieuOfCashRepayment" name="InvestmentInLieuOfCashRepayment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" 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_UnitNotesAggregateFaceAmount" name="UnitNotesAggregateFaceAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
</schema>
