Well there is d9vk and dxvk which translates directX 9, 10 and 11 to Vulkan and vkd3d which translates direct3D (directX 12) to Vulkan. So it’s all Vulkan on the Linux side, Proton ‘just’ translates one of these into Vulkan or passes it through if the game is already Vulkan.

I don’t think there is a native (to GTA IV) Vulkan renderer in game though and therefore there isn’t anything more you can or have to do on Linux.

On Windows, some older games actually get performance improvements from the translation to Vulkan.