I am actively developing a vulkan backend at least to the point where if apple removes OpenGL we can pretty quickly get something going for VTK. It will use MoltenVK to drive Metal. It exists and passes some tests. It is currently missing an absurd number of features but it is what I am working on right now and I plan to continue fleshing it out for at least a couple more weeks if not more. My current goal is risk mitigation so that should something happen we have a reasonable head start on addressing it.
It would be nice sometime in the next few months to start preparing/examinging what changes would be required in PV to use vulkan. Right now PV directly interfaces with some OpenGL classes and that would need to be rersolved.