The advantage of vulkan/dx12 for VR is massive. It has been a decade games are in a large part badly CPU bound by dx and driver, rendering 2 views make it double worse. The new API is incredibly ligther for the CPU and provide new features to be smarter on how we send frames :) plus the explicit multigpu that is handy for VR.
On the GPU, we may save a little because we ditch a lot of automatic safeguard and because we may go more fine grain thanks to the new flexibility ( see recent frostbite talk or assassin creed 4 one, everyone is moving to gpu culling and ditch umbra :p )
