I'd like to add that as a developer, I could really benefit from Vulkan support. Being able to stay on Mac OS while I code in Visual Studio inside of the Windows VM, compile on both platforms, and test on both platforms, would be a major, major, major productivity booster. Even if I don't get every feature or exact accuracy, that's fine. As long as it's stable and lets me run dev tools, I'm good.
There's one application in particular that I'd definitely like to see work well: RenderDoc. It's an invaluable development tool for introspecting and debugging a graphics pipeline. I'm willing to bet that Parallels' developers probably use it!
MoltenVK is a Vulkan -> Metal API translation layer for macOS that already works quite well. Perhaps this could be used as a springboard for Vulkan support within Windows guests on a macOS host. Although I'm sure you're already aware of this, I figured it's worth mentioning.