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	  <link:definition>0018 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0019 - Disclosure - Fair Value Measurements (Tables)</link:definition>
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	  <link:definition>0020 - Disclosure - Prepaid Expenses (Tables)</link:definition>
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	  <link:definition>0021 - Disclosure - Accrued Expenses (Tables)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0022 - Disclosure - Convertible Promissory Notes and Other Notes Payable (Tables)</link:definition>
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	  <link:definition>0023 - Disclosure - Capital Stock (Tables)</link:definition>
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	  <link:definition>0024 - Disclosure - 1. History and Organization (Details Textuals)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0025 - Disclosure - 2. Basis of Presentation and Going Concern (Details Textuals)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0026 - Disclosure - Summary of Significant Accounting Policies (Details)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0027 - Disclosure - 3. Summary of Significant Accounting Policies (Details1)</link:definition>
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	  <link:definition>0028 - Disclosure - Summary of Significant Accounting Policies (Details Textuals)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0029 - Disclosure - Fair Value Measurements (Details)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0030 - Disclosure - 5. Prepaid Expenses (Details)</link:definition>
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	  <link:usedOn>link:calculationLink</link:usedOn>
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	  <link:definition>0031 - Disclosure - 6. Accrued Expenses (Details)</link:definition>
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	  <link:definition>0032 - Disclosure - 7. Convertible Promissory Notes and Other Notes Payable (Details)</link:definition>
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	  <link:definition>0033 - Disclosure - 7. Convertible Promissory Notes and Other Notes Payable (Details Narrative)</link:definition>
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	  <link:definition>0034 - Disclosure - 10. Licensing and Collaborative Agreements (Details Textuals)</link:definition>
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	  <link:definition>0035 - Disclosure - Capital Stock (Details)</link:definition>
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	  <link:definition>0036 - Disclosure - Capital Stock (Details 1)</link:definition>
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	  <link:definition>0037 - Disclosure - 11. Related Party Transactions (Details Textuals)</link:definition>
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	  <link:definition>0038 - Disclosure - Subsequent Events (Details Textuals)</link:definition>
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    <element id="VSTA_LicensingMember" name="LicensingMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NotesPayableToRelatedPartiesMember" name="NotesPayableToRelatedPartiesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CurrentPortionFourMember" name="CurrentPortionFourMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Note1Member" name="Note1Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Note2Member" name="Note2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNote1Member" name="SeniorNote1Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNote2Member" name="SeniorNote2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNote3Member" name="SeniorNote3Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNote4Member" name="SeniorNote4Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNote5Member" name="SeniorNote5Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SeniorNonCurrentMember" name="SeniorNonCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SevenPercentNotesPayableAprilTwoThousandElevenMember" name="SevenPercentNotesPayableAprilTwoThousandElevenMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SevenPercentNoteCurrentPortionMember" name="SevenPercentNoteCurrentPortionMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SevenPercentNoncurrentMember" name="SevenPercentNoncurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_MAndFNoteBMember" name="MAndFNoteBMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <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" />
    <element id="VSTA_ServiceProviderNotesTotalMember" name="ServiceProviderNotesTotalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <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" />
    <element id="VSTA_VendorNoncurrentMember" name="VendorNoncurrentMember" 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_RelatedPartiesCurrentMember" name="RelatedPartiesCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_RelatedPartiesAfterDiscountMember" name="RelatedPartiesAfterDiscountMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ServiceProvidersAfterDiscountMember" name="ServiceProvidersAfterDiscountMember" 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_SeniorNoteNoncurrentMember" name="SeniorNoteNoncurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SevenPercentNoteMember" name="SevenPercentNoteMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SevenPercentNoteCurrentMember" name="SevenPercentNoteCurrentMember" 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_ServiceProviderTotalMember" name="ServiceProviderTotalMember" 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_ServiceNotesCurrentMember" name="ServiceNotesCurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_SeniorNotesTotalMember" name="SeniorNotesTotalMember" 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_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_Warrant11Member" name="Warrant11Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant12Member" name="Warrant12Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant13Member" name="Warrant13Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantModificationMember" name="WarrantModificationMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AugustFinancingMember" name="AugustFinancingMember" 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_LiabilitiesAndStockholdersDeficitAbstract" name="LiabilitiesAndStockholdersDeficitAbstract" nillable="true" xbrli:periodType="instant" 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" />
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    <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" />
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    <element id="VSTA_AcquiredInProcessResearchAndDevelopment" name="AcquiredInProcessResearchAndDevelopment" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ChangeInPutAndNoteExtensionOptionAndWarrantLiabilities" name="ChangeInPutAndNoteExtensionOptionAndWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WeightedAverageSharesUsedInComputing" name="WeightedAverageSharesUsedInComputing" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_AmortizationOfImputedDiscountOnNonInterestBearingNotes" name="AmortizationOfImputedDiscountOnNonInterestBearingNotes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AmortizationOfDiscountsOn77.5And10Notes" name="AmortizationOfDiscountsOn77.5And10Notes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_AmortizationOfDiscountsOnFebruary201212ConvertibleNotes" name="AmortizationOfDiscountsOnFebruary201212ConvertibleNotes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_FairValueOfCommonStockIssuedForNoteTermModification" name="FairValueOfCommonStockIssuedForNoteTermModification" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InterestIncomeOnNoteReceivableForStockPurchase" name="InterestIncomeOnNoteReceivableForStockPurchase" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConsultingServicesByRelatedPartiesSettledByIssuingPromissoryNotes" name="ConsultingServicesByRelatedPartiesSettledByIssuingPromissoryNotes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_NetProceedsFromIssuanceOf20062007NotesAndWarrants" name="NetProceedsFromIssuanceOf20062007NotesAndWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProceedsFromIssuanceOf7NotesPayable" name="ProceedsFromIssuanceOf7NotesPayable" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NetProceedsFromIssuanceOfAugust2010ShortTermNotesAndWarrants" name="NetProceedsFromIssuanceOfAugust2010ShortTermNotesAndWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_HistoryAndOrganizationTextBlock" name="HistoryAndOrganizationTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrepaidExpensesTextBlock" name="PrepaidExpensesTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_LicensingAndCollaborativeAgreementsTextBlock" name="LicensingAndCollaborativeAgreementsTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_SummaryOfSignificantAccountingPoliciesPoliciesAbstract" name="SummaryOfSignificantAccountingPoliciesPoliciesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantLiabilityPolicyTextBlock" name="WarrantLiabilityPolicyTextBlock" 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_PrepaidExpensesTablesAbstract" name="PrepaidExpensesTablesAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrepaidExpensesTableTextBlock" name="PrepaidExpensesTableTextBlock" 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_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_WarrantExercisesAndGrantsTableTextBlock" name="WarrantExercisesAndGrantsTableTextBlock" 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_OutstandingShares" name="OutstandingShares" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" 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_CashExpenditureMaximum" name="CashExpenditureMaximum" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CashExpenditureMinimum" name="CashExpenditureMinimum" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SharePurchaseObligation" name="SharePurchaseObligation" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SharePurchaseObligationCashConsideration" name="SharePurchaseObligationCashConsideration" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InitialShareIssuance" name="InitialShareIssuance" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" 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_SummaryOfSignificantAccountingPoliciesDetails1Abstract" name="SummaryOfSignificantAccountingPoliciesDetails1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NumeratorAbstract" name="NumeratorAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DenominatorAbstract" name="DenominatorAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AssumedConversionOfDilutiveSecurities" name="AssumedConversionOfDilutiveSecurities" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PotentiallyAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" name="PotentiallyAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" 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_OutstandingWarrantsToPurchaseCommonStock" name="OutstandingWarrantsToPurchaseCommonStock" 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_SummaryOfSignificantAccountingPoliciesDetailsTextualsAbstract" name="SummaryOfSignificantAccountingPoliciesDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AdditionalSharebasedCompensationCosts" name="AdditionalSharebasedCompensationCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" 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_PrepaidExpensesDetailsAbstract" name="PrepaidExpensesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" 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_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_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_GrossProceedsConversionNote" name="GrossProceedsConversionNote" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedInterestRatePerShare" name="AccruedInterestRatePerShare" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" 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_AmountPaidForUhnRoyalties" name="AmountPaidForUhnRoyalties" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AwardGivenToSupportDevelopment" name="AwardGivenToSupportDevelopment" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AwardedAmountIncreasedToSupportDevelopment" name="AwardedAmountIncreasedToSupportDevelopment" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UnsecuredPromissoryNoteIssued2" name="UnsecuredPromissoryNoteIssued2" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantsExercisable" name="WarrantsExercisable" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" 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_ExpirateDateRange" name="ExpirateDateRange" nillable="true" xbrli:periodType="instant" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardWarrantsExercisableNumber" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardWarrantsExercisableNumber" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" 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_MonhtlyInstallments" name="MonhtlyInstallments" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CrlNoteInterestRate" name="CrlNoteInterestRate" nillable="true" xbrli:periodType="duration" type="num:percentItemType" 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_SubsequentEventsDetailsTextualsAbstract" name="SubsequentEventsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantsExercised" name="WarrantsExercised" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProceedsFromIssuance" name="ProceedsFromIssuance" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ModifiedWarrantsExercised" name="ModifiedWarrantsExercised" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProfessionalServicesPaymentValue" name="ProfessionalServicesPaymentValue" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RestrictedCommonStock" name="RestrictedCommonStock" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
</schema>
