I am not entirely sure what is meant with "composited Vulkan swapchains", but if it is related to implementing a compositor in Wine
Yes, I mean implementing compositing in Wine. For instance for proper child window clipping, GDI interop, DPI scaling, cross-process rendering and so on. Some of it may be delegated to the host compositing features, and we need to try to keep using as much possible for performance reasons, and as the most common cases that don't require compositing. But many exotic use cases can't reliably be implemented through the host composition, especially not in a sane way considering all the platforms we need to support.
wouldn't it make more sense to implement this on top of D3D?
I don't see how it would, this also includes client GL/VK/GDI interop, not just D3D. Pretty much has to live on the unix side, or be delegated to some other process but that's mostly the same. -- https://gitlab.winehq.org/wine/wine/-/merge_requests/11058#note_143326