Direct3D is a subset of DirectX, which also includes a bunch of other Windows APIs named Direct[foo] for things like sound. DXVK is a port of WINE’s Direct3D interface to Vulkan (it would previously have used OpenGL, I presume). Since DXVK is a first cut at moving to a new lib, I would bet the internals are an incompletely-documented mess, which presumably is exactly what they mean to clean up.