Comment No. (Score 3, Interesting) 7
It is not cross-API compatible because it is an API. It's implemented on multiple platforms and it's only supported on platforms it's implemented for.
Also, you seem to be forgetting there are a lot of API specific functions functions: e.g. vkCreateWaylandSurfaceKHR, vkCreateWin32SurfaceKHR, vkCreateXcbSurfaceKHR, vkCreateXlibSurfaceKHR.