I think -- and I could be wrong here -- that gfx-rs aims to eventually implement Vulkan on D3D12 in addition to Metal, while MoltenVK only aims to work on Metal.
I don't know about the MoltenVK branding specifically, but Khronos does plan a "Universally portable subset" of Vulkan with tools to target both Metal and D3D12 from one rendering codebase and common SPIR-V shaders.
The project is call Vulkan Portability Initiative:
If I understood correctly, gfx-rs aims to provide an implementation for that initiative. Khronos only announced it in concept, they didn't implement it.
Technically Windows doesn't support Vulkan at all, but the openness of the win32 platform allowed Khronos to define a "backdoor" interface where the application ignores the Windows graphics stack and instead gets a Vulkan context by talking to the graphics driver directly. The UWP sandbox makes this impossible so you're stuck with the native Direct3D APIs.
Statistically, nobody actually builds real UWP apps, so it is largely an academic point. But if you're out of the sandbox, I fail to see a good reason not to just build a regular old Win32 app the way it has always been done.
The project is call Vulkan Portability Initiative:
https://www.khronos.org/blog/khronos-announces-the-vulkan-po...