You can't "assume Vulkan exists". Any pre-2016 hardware won't have proper hardware support for Vulkan and that's a lot of hardware still in use. Software renderers are unworthy of any serious consideration due to the perfomance drawbacks.

Just use OpenGL. I don't know when this trend to overcomplicate everything using Vulkan began, but I hate it.

Nvidia had a driver for Vulkan for Kepler which launched in 2012, AMD had support all the way back to GCN 1.0 (also 2012). Intel did have issues supporting it, I can't recall if it was for hardware reasons or just lack of desire for a driver.

Vulkan has substantial advantages for multi-threaded code, as well as exposing the underlying asynchronous nature of running code on the GPU. The kind of thing you want to be able to control with a desktop compositor where controlling vsync and present timing is very important.

There are Xorg and XLibre for pre-2016 hardware.