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Net Loss per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Shares Outstanding (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Convertible preferred stock [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Potentially dilutive securities excluded from computation of diluted shares outstanding   10,491,140us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_ConvertiblePreferredStockMember		 5,991,757us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_ConvertiblePreferredStockMember
Warrants to purchase convertible preferred stock [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Potentially dilutive securities excluded from computation of diluted shares outstanding   173,912us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= vsar_WarrantsToPurchaseConvertiblePreferredStockMember		 [1] 173,912us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= vsar_WarrantsToPurchaseConvertiblePreferredStockMember		 [1]
Options to purchase common stock [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Potentially dilutive securities excluded from computation of diluted shares outstanding 2,723,366us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= vsar_OptionsToPurchaseCommonStockMember		 1,403,655us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= vsar_OptionsToPurchaseCommonStockMember		 462,471us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= vsar_OptionsToPurchaseCommonStockMember
Convertible notes [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Potentially dilutive securities excluded from computation of diluted shares outstanding     885,024us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleNotesPayableMember
Restricted stock units [Member]      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Potentially dilutive securities excluded from computation of diluted shares outstanding 185,514us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_RestrictedStockUnitsRSUMember		    
[1] Assumes exercise of warrants to purchase convertible preferred stock at $5.17 per share.