Thanks to the modding community, MD's FideliltyFX Super Resolution (AMD FSR) is now usable in SteamVR and Vulkan compatible games if you are willing to follow some steps and implement it manually. Due to the open-source nature of FSR, this has allowed gamers to effectively build an FSR "patch" for both SteamVR and Vulkan.
The Vulkan modification was built by Georg Lehmann (@DadSchoorse); and works in both the dxvk shader and Steam's Proton layer by installing FSR into FSHack -- a technology that allows for lower in-game resolutions without changing the native resolution of your monitor.
This mod will allow you to use FSR in almost any Vulkan-supported game and gives you more control than AMD's normal FSR implementations. You can choose any resolution you want to upscale from, and choose a sharpness value to your own liking. 0 is maximum sharpness, and any higher number results in lower sharpening of the game.
But there are several caveats that can make adding FSR a headache, some of which include automatic upscalers which don't work at all with FSR, and the patchset can be a headache on its own to install.
You can check out the Vulkan FSR mod here.
SteamVR implementation, built by Frydrych Holger (@fholger), is a bit simpler to work with. All you need to do is download a modified .dll file and install it directly into the game's file directory.
The FSR implementation here is the same as Vulkan, allowing you to change resolutions specifically and change sharpness at will.
You can check out the SteamVR FSR mod here.
The modding community has shown us how powerful FSR's open-source nature can be, making it easy to add FSR into any game you want. This should hopefully accelerate FSR's adoption rates even more and give DLSS some extra competition.