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    <element id="VSTA_ConvertiblePromissoryNotesAndAccruedInterest" name="ConvertiblePromissoryNotesAndAccruedInterest" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedDividendsOnSeriesBPreferredStock" name="AccruedDividendsOnSeriesBPreferredStock" 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_FairValueOfSeriesBPreferredStockGrantedForServices" name="FairValueOfSeriesBPreferredStockGrantedForServices" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_PrepaidExpensesTextBlock" name="PrepaidExpensesTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantLiabilityPolicyTextBlock" name="WarrantLiabilityPolicyTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_PrepaidExpensesTableTextBlock" name="PrepaidExpensesTableTextBlock" nillable="true" xbrli:periodType="duration" type="nonnum:textBlockItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_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_LegalFees1" name="LegalFees1" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" type="xbrli:monetaryItemType" 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_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_WarrantSharesIssuableUponExerciseNotePurchaseAndExchangeAgreement" name="WarrantSharesIssuableUponExerciseNotePurchaseAndExchangeAgreement" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_ConvertiblePromissoryNotesAndAccruedInterest1" name="ConvertiblePromissoryNotesAndAccruedInterest1" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_WarrantOrRightOutstanding" name="WarrantOrRightOutstanding" nillable="true" xbrli:periodType="duration" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ExpirateDateRange" name="ExpirateDateRange" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_SeriesPreferredStockIssuedAndOutstanding" name="SeriesPreferredStockIssuedAndOutstanding" nillable="true" xbrli:periodType="instant" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_PostModificationMember" name="PostModificationMember" 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" />
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    <element id="VSTA_RelatedPartiesAfterDiscountMember" name="RelatedPartiesAfterDiscountMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_MFNoteAMember" name="MFNoteAMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_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_Warrant2Member" name="Warrant2Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant3Member" name="Warrant3Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant4Member" name="Warrant4Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant5Member" name="Warrant5Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant6Member" name="Warrant6Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Warrant7Member" name="Warrant7Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_USNIHMember" name="USNIHMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_UHNMember" name="UHNMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_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_TenPercentNotesMember" name="TenPercentNotesMember" 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" />
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    <element id="VSTA_PreMod3Member" name="PreMod3Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_Price1440to36Member" name="Price1440to36Member" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Unit2014ProceedsMember" name="Unit2014ProceedsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_Unit2013ProceedsMember" name="Unit2013ProceedsMember" nillable="true" xbrli:periodType="duration" type="nonnum:domainItemType" substitutionGroup="xbrli:item" />
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    <element id="VSTA_PrepaidExpensesDetailsAbstract" name="PrepaidExpensesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_AccruedExpensesDetailsAbstract" name="AccruedExpensesDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableDetailsAbstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetailsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_ConvertiblePromissoryNotesAndOtherNotesPayableDetails2Abstract" name="ConvertiblePromissoryNotesAndOtherNotesPayableDetails2Abstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
    <element id="VSTA_CapitalStockDetails2Abstract" name="CapitalStockDetails2Abstract" 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_RelatedPartyTransactionsDetailsTextualsAbstract" name="RelatedPartyTransactionsDetailsTextualsAbstract" abstract="true" nillable="true" xbrli:periodType="duration" type="xbrli:stringItemType" substitutionGroup="xbrli:item" />
</schema>
