That sounds like the typical black flicker that can sometimes happen if the GPU is overloaded. Not just with vorpX, but generally with the Rift.
First make sure that you have the latest graphics driver and runtime 0.8 installed. In case of nVidia older dirvers showed this behaviour more often than newer ones.
If you still experience this afterwards, try to reduce the workload on the GPU by lowering graphics options in your game or/and lowering the resolution you play the game with. A few vorpX settings you can adjust to reduce the workload are listed in the performance optimization guide in the vorpX help.