I do see that the option doesn't seem to do anything, but it also seems the same with wined3d. Will check Windows here later.
It appears that this is a game bug as the option doesn't do anything with the native game on Windows either.
Was your game set to windowed? Vsync in D3D9 only works in fullscreen.
Tried both
Strange. I downloaded the demo version from Steam and I can see Vsync toggle working correctly.
Can't get the demo working on Linux at the moment.
But i can see that it is d3d9 instead of d3d9Ex and looking at SteamDB it seems the full version received a big update here back in March that the demo didn't, so the latter is probably not latest version.
Edit: Yes vsync works fine if i use the version pre March update (still d3d9Ex).
Edit 2: Made a note about it on PCGamingWiki
So it's a game bug according to @Blisto91.
proton 9.0-2x1 2024-08
The vsync switch option is not working (menu or in-game). This game uses D3D9Ex (don't know if that makes a difference for DXVK's implementation). I'm attaching a trace with the vsync on/off click. I've tried on X11 and Wayland.
Software information
System information
Apitrace file(s)
Log files
If more information is needed, let me know.