There is still no functional VR media player for linux, none of the Windows ones work with Proton and the official SteamVR Media Player has no linux release.
[0] https://github.com/frostworx/steamtinkerlaunch
[1] https://github.com/frostworx/steamtinkerlaunch#Side-by-Side-...
Edit: I should say side-by-side mode works for games with it builtin (like Trine) but can also use shader injection to emulate this which can work pretty well too.
VLC also works for me from the start, but seems to lag at higher resolutions, hence fiddling to get DirectShow working.