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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
 1 Months Ended 3 Months Ended 12 Months Ended 73 Months Ended 1 Months Ended
Mar. 31, 2014
Mar. 31, 2014
Dec. 31, 2014
Segment
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Feb. 28, 2014
Significant Accounting Policies [Line Items]              
Accumulated deficit     $ 111,259,000us-gaap_DevelopmentStageEnterpriseDeficitAccumulatedDuringDevelopmentStage $ 53,746,000us-gaap_DevelopmentStageEnterpriseDeficitAccumulatedDuringDevelopmentStage   $ 111,259,000us-gaap_DevelopmentStageEnterpriseDeficitAccumulatedDuringDevelopmentStage  
Working capital     166,000,000vsar_WorkingCapital     166,000,000vsar_WorkingCapital  
Number of operating segment     1us-gaap_NumberOfOperatingSegments        
Reverse stock split     1-for-11.5        
Cash and cash equivalents maturity period     Three months or less        
Cash and cash equivalents of restricted cash held for credit cards     100,000us-gaap_RestrictedCashAndCashEquivalents 100,000us-gaap_RestrictedCashAndCashEquivalents   100,000us-gaap_RestrictedCashAndCashEquivalents  
Impairments of long-lived assets     0us-gaap_ImpairmentOfLongLivedAssetsHeldForUse 0us-gaap_ImpairmentOfLongLivedAssetsHeldForUse   0us-gaap_ImpairmentOfLongLivedAssetsHeldForUse  
Inception date     Dec. 10, 2008        
Number of securities issued upon exercise of warrants 158,179us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights 158,179us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights          
Remeasurement of outstanding warrants 173,910vsar_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionOutstandingInPeriodIntrinsicValue            
Warrants to purchase convertible preferred stock, exercise price $ 5.17us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 $ 5.17us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1          
Convertible preferred stock shares automatically converted into common stock 15,876,104us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion 15,876,104us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion          
Purchase price of convertible preferred stock     64,793,000us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock 29,956,000us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock 5,765,000us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock 120,113,000us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock  
Deemed dividend related to beneficial conversion feature of convertible preferred stock   25,559,000us-gaap_OtherPreferredStockDividendsAndAdjustments 25,559,000us-gaap_OtherPreferredStockDividendsAndAdjustments     25,559,000us-gaap_OtherPreferredStockDividendsAndAdjustments  
Series D convertible preferred stock [Member]              
Significant Accounting Policies [Line Items]              
Remeasurement of preferred stock             9,600,000vsar_ReMeasurementOfPreferredShares
/ us-gaap_StatementClassOfStockAxis
= vsar_SeriesDConvertiblePreferredStockMember
Series E convertible preferred stock [Member]              
Significant Accounting Policies [Line Items]              
Convertible preferred stock shares automatically converted into common stock             48,758,857us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion
/ us-gaap_StatementClassOfStockAxis
= vsar_SeriesEConvertiblePreferredStockMember
Convertible preferred stock, Purchase price             $ 1.128vsar_ConvertiblePreferredStockPurchasedPricePerShare
/ us-gaap_StatementClassOfStockAxis
= vsar_SeriesEConvertiblePreferredStockMember
Purchase price of convertible preferred stock             55,000,000us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock
/ us-gaap_StatementClassOfStockAxis
= vsar_SeriesEConvertiblePreferredStockMember
Number of common stock automatically converted from convertible preferred stock upon completion of the Company's IPO             4,239,984us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
/ us-gaap_StatementClassOfStockAxis
= vsar_SeriesEConvertiblePreferredStockMember
Deemed dividend related to beneficial conversion feature of convertible preferred stock             25,600,000us-gaap_OtherPreferredStockDividendsAndAdjustments
/ us-gaap_StatementClassOfStockAxis
= vsar_SeriesEConvertiblePreferredStockMember
IPO [Member]              
Significant Accounting Policies [Line Items]              
Warrants to purchase convertible preferred stock, exercise price $ 21.00us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember		 $ 21.00us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
/ us-gaap_SubsidiarySaleOfStockAxis
= us-gaap_IPOMember		          
Remeasurement [Member]              
Significant Accounting Policies [Line Items]              
Expense related to change in fair value 2,300,000us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= vsar_RemeasurementMember		            
Reclassified to additional paid in capital $ 2,800,000vsar_ReclassificationOfAdjustedFairValueOfWarrantsIntoAdditionalPaidInCapital
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= vsar_RemeasurementMember		            
Minimum [Member]              
Significant Accounting Policies [Line Items]              
Property and equipment estimated useful lives     3 years        
Maximum [Member]              
Significant Accounting Policies [Line Items]              
Property and equipment estimated useful lives     5 years        
Percentage for the amount of benefit realized upon ultimate settlement     50.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 50.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements
/ us-gaap_RangeAxis
= us-gaap_MaximumMember