Around a week after I started, and the game is playable on Vulkan and behaves just like the 3.x version AFAICT - if you pick the loaded map, not the generated map:
https://github.com/Zireael07/FreeRoamRoguelikeRacerPrototype/tree/4.x
I also did some more minor improvements on a yet another separate branch as well as I've tried putting lights around the map, now that Vulkan effectively has no limit on the amount of lights, they can be used to light up the map at night-time
The single big issue to solve is to figure out what happens wrong with the mapgen...