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Oil and natural gas properties (under successful efforts accounting) (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Extractive Industries [Abstract]    
Oil and natural gas properties $ 3,767,267vyey_OilandGasPropertySuccessfulEffortsMethodNetofImpairment $ 2,838,573vyey_OilandGasPropertySuccessfulEffortsMethodNetofImpairment
Less: accumulated depletion (1,780,232)us-gaap_OilAndGasPropertySuccessfulEffortMethodAccumulatedDepreciationDepletionAndAmortization (1,942,380)us-gaap_OilAndGasPropertySuccessfulEffortMethodAccumulatedDepreciationDepletionAndAmortization
Total oil and gas properties, net $ 1,987,035us-gaap_OilAndGasPropertySuccessfulEffortMethodNet $ 896,193us-gaap_OilAndGasPropertySuccessfulEffortMethodNet