I've got error saying "Fallout: New Vegas does not appear to be installed and the installer could not be found.". After research I found this solution on internet:

In Steam folder in (...)/steamapps/compdata/22490/pfx I edited system.reg file and at the very end append (with an empty line between it and the last entry):

[Software\\Wow6432Node\\Bethesda Softworks\\FalloutNV] 1627669000
#time=1d7856f0ad32d72
"Installed Path"="C:\Program Files (x86)\Steam\steamapps\common\Fallout New Vegas\"

And it worked! It doesn't even need to have "enplczru" or any of that, and actually it works ONLY without this.

• some ground textures rarely can load in very low scale, causing them to be blured. This only happened in exteriors so far, and can be easliy fixed with reloading game (exiting to main menu/desktop; loading save; walking in to other location or fast traveling).
• in far distance some texture slightly flickers, but I'm not sure if it's Proton fault or just Bethesda's buggy game engine lol

• 10 fps slower compared to playing on Windows (on high settings)
• too many ragdoll objects can cause drop fps to zero for a second or few (this might be issue only because of my low-end hardware, tho I didn't notice that much of fps drops playing on Windows)