So it's called Vulkan, not Vulcan. That always bothers me when people spell it wrong. Anyway, the API should not make a huge difference if it is implemented properly. VR just uses a stereo render (which can be done in any API, even DirectX9) and special VR-specific APIs for the tracking functionality, which is just C/C++ code and doesn't have anything to do with rendering. So I don't see why there would be any problem.