I'm experiencing bad performance issues (as many people are!) and I saw on the Steam discussion forum for the game that users are reporting better performance using Vulkan.

They force it with a Steam launch option:

[-64bit -vulkan -high ]

I've experimented with various versions of the above on GOG launch parameters but the game keeps using DX12.

Does anybody know how to force GOG to launch the game using Vulkan?

edit: nevermind