There's Experimental Work On A Vulkan Renderer For KDE's KWin
Over the past week Fredrik Höglund has begun work on KWin Vulkan support so this low-level, high-performance graphics API could be used for compositing rather than OpenGL. So far he charted out a lot of the fundamental Vulkan code and the necessary infrastructure work along with some basic features like for being able to render window shadows and porting some other window effects over to Vulkan.
The work in its still very early stages can be found via this Git branch but for now likely wouldn't be useful or do much for end-users not wanting to help out with development. It will certainly be interesting to see where it goes. It's worth pointing out the work so far is being led by Fredrik Höglund who is not a no-name developer but rather a longtime KDE contributor and has also contributed to Mesa routinely as well. Back in 2015 is when KWin maintainer Martin Flöser discounted the idea of Vulkan support for KWin, but he also didn't think that Vulkan drivers would mature that quickly either.
Anyhow, it will certainly be interesting to see what comes of this Git branch for KWin Vulkan support. So far we haven't seen GNOME's Mutter compositor / window manager pursue any Vulkan support, but the GTK 4.0 tool-kit does have a maturing Vulkan renderer. For the Qt5 tool-kit, there is the Vulkan enablers as of Qt 5.10 but not yet any mainline support for rendering of Qt interfaces with Vulkan.