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    <element id="VSTA_AdditionalInvestorRelationsServicesMember" name="AdditionalInvestorRelationsServicesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantModificationsMember" name="WarrantModificationsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ForegoingExercisesMember" name="ForegoingExercisesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PlatinumMember" name="PlatinumMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_QualifiedFinancingMember" name="QualifiedFinancingMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CatoHoldingCompanyMember" name="CatoHoldingCompanyMember" 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_CatoBioVenturesMember" name="CatoBioVenturesMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_USNationalInstitutesOfHealthMember" name="USNationalInstitutesOfHealthMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NIHGrantRevenueMember" name="NIHGrantRevenueMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <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" />
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    <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" />
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    <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" />
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    <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" />
    <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_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" />
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    <element id="VSTA_UnrelatedPartiesNoncurrentMember" name="UnrelatedPartiesNoncurrentMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <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" />
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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" />
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    <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" />
    <element id="VSTA_ServiceProvidersTotalMember" name="ServiceProvidersTotalMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_RelatedPartiesTotalMember" name="RelatedPartiesTotalMember" 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_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" />
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    <element id="VSTA_PrivatePlacement2013Member" name="PrivatePlacement2013Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_Note4Member" name="Note4Member" 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" />
    <element id="VSTA_CatoMember" name="CatoMember" 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_SeniorNote6Member" name="SeniorNote6Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_TenPercentNotesMember" name="TenPercentNotesMember" 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_PlatinumModificationSeriesAMember" name="PlatinumModificationSeriesAMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PlatinumModificationRangeHighMember" name="PlatinumModificationRangeHighMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_PreMod2Member" name="PreMod2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PostMod2Member" name="PostMod2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PreMod3Member" name="PreMod3Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PostMod3Member" name="PostMod3Member" 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" />
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    <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_ChangeInPutAndNoteExtensionOptionAndWarrantLiabilities" name="ChangeInPutAndNoteExtensionOptionAndWarrantLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <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_LossOnSettlementsOfAccountsPayable" name="LossOnSettlementsOfAccountsPayable" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_LossOnEarlyExtinguishmentOfDebt" name="LossOnEarlyExtinguishmentOfDebt" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_FairValueOfCommonStockIssuedForNoteTermModification" name="FairValueOfCommonStockIssuedForNoteTermModification" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_NetProceedsFromIssuanceOf7ConvertibleNotes" name="NetProceedsFromIssuanceOf7ConvertibleNotes" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NetProceedsFromIssuanceOf10ConvertibleNotesAndWarrants" name="NetProceedsFromIssuanceOf10ConvertibleNotesAndWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NetProceedsFromIssuanceOfPlatinumNotesAndWarrants" name="NetProceedsFromIssuanceOfPlatinumNotesAndWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_NetProceedsFromIssuanceOf20082010NotesAndWarrants" name="NetProceedsFromIssuanceOf20082010NotesAndWarrants" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <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" />
    <element id="VSTA_NetProceedsFromIssuanceOfFebruary201212ConvertibleNotesAndWarrants" name="NetProceedsFromIssuanceOfFebruary201212ConvertibleNotesAndWarrants" 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_PrepaidExpensesTextBlock" name="PrepaidExpensesTextBlock" 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_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_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_ComponentsOfDiscountAndInterestRatesTableTextBlock" name="ComponentsOfDiscountAndInterestRatesTableTextBlock" 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_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" />
    <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_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_SummaryOfSignificantAccountingPoliciesDetailsAbstract" name="SummaryOfSignificantAccountingPoliciesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpenseWarrantsGrantedToOfficer" name="ExpenseWarrantsGrantedToOfficer" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <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_WarrantsGrantedToOfficersAndDirectorExpense" name="WarrantsGrantedToOfficersAndDirectorExpense" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_TotalGeneralAndAdministrativeExpenseRelatedToStockbasedCompensationExpense" name="TotalGeneralAndAdministrativeExpenseRelatedToStockbasedCompensationExpense" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_SummaryOfSignificantAccountingPoliciesDetails2Abstract" name="SummaryOfSignificantAccountingPoliciesDetails2Abstract" 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_WarrantsExcludedFromComputationOfEarningsPerShareAmount" name="WarrantsExcludedFromComputationOfEarningsPerShareAmount" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" 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_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_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_ConvertiblePromissoryNotesAndOtherNotesPayableDetails1Abstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetails1Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DebtInstrumentDomain" name="DebtInstrumentDomain" abstract="true" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DiscountAttributableToFairValueOfWarrant" name="DiscountAttributableToFairValueOfWarrant" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DiscountAttributableToBeneficialConversionFeature" name="DiscountAttributableToBeneficialConversionFeature" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_InceptionDateCarryingValue" name="InceptionDateCarryingValue" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DecreaseinDerivativeLiabilitiesQuarter" name="DecreaseinDerivativeLiabilitiesQuarter" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DecreaseinDerivativeLiabilitiesThreeMonth" name="DecreaseinDerivativeLiabilitiesThreeMonth" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpectedTermYears" name="ExpectedTermYears" nillable="true" xbrli:periodType="instant" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_DividendRate" name="DividendRate" nillable="true" xbrli:periodType="instant" type="num:percentItemType" 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_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_PeriodicResearchAndDevelopmentArrangementWithFederalGovernmentCustomerFundingToOffsetCostsIncurred" name="PeriodicResearchAndDevelopmentArrangementWithFederalGovernmentCustomerFundingToOffsetCostsIncurred" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AwardUponAgreedMilestones" name="AwardUponAgreedMilestones" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" 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_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_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_OwnshareLendingArrangementPerSharePrice" name="OwnshareLendingArrangementPerSharePrice" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_OwnshareLendingArrangementSharesOutstandingCommitmentValue" name="OwnshareLendingArrangementSharesOutstandingCommitmentValue" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" 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_RestrictedCommonStock" name="RestrictedCommonStock" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExercisePriceAtGrantDate" name="ExercisePriceAtGrantDate" nillable="true" xbrli:periodType="instant" type="num:perShareItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ProbabilityOfIssuance" name="ProbabilityOfIssuance" nillable="true" xbrli:periodType="instant" type="num:percentItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ShareBasedCompensationArrangementByShareBasedPaymentAwardModifiedNonOptionEquityInstrumentsExercised" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardModifiedNonOptionEquityInstrumentsExercised" nillable="true" xbrli:periodType="duration" 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_SubsequentEventsDetailsTextualsAbstract" name="SubsequentEventsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
</schema>
