Although I don't know if this applies to the VR version (if it differs), other users have played the standard game with "-force-glcore45" launch option, which should perform better than having to translate from D3D11 -> vulkan/OGL.

-force-glcore45

With this option, game does not display in the headset, only on-screen, and there is no head tracking. (EDIT: "no head tracking" meaning that, unlike when VR is working, on-screen game output does not follow headset movement)

The first start after install is butter smooth, subsequent have extremely poor FPS (2~4fps maybe ?). Some caching gone wrong ?

Looking more carefully, this is wrong: when keeping the headset still, I can see the waves in menu background scene are moving smoothly. It is rather the headset tracking which is very shaky (either position is updated at most 4 times per second, or maybe it is the orientation snapping back to a previous position... unsure).

Also, while doing back-and-forth with steamvr configuration page open, I noticed that there were fewer missed frames when I was not looking into the headset... It appears to be the waves which are somehow causing the stutter: looking up far enough so the waves are off-screen and not rendered, headset suddenly tracks smoothly.

Steam - System Information

I have a similair/same experience.

I can get subnautica to launch in vr and with headtracking working but only if I remove all launch options. However xbox-controller input is not working. Neither is mouse or keyboard.

And I get the same graphical glitches (some shaders are transparent) as when I run subnautica normally without "-force-opengl" or "-force-glcore45".

It launches with Proton 4.11-13 but not with Proton 5.0-9 or Proton 5.13-1, with this window appearing when launching in VR (launching in pancake mode works):

...which I should have done from the beginning, dumb me. Sorry, @kisak-valve. Here it is, in all its glory, the steam-264710.log file!
steam-264710.log

@Patola It seems that Subnautica VR was disabled on purpose in DXVK 1.7.2 https://github.com/doitsujin/dxvk/commit/07fee4a0e538fb95d40809c3048b591cf5268f8f That DXVK commit was included in Proton 5.0-2.

I couldn't find a specific DXVK issue pertaining to that commit, but it could have been a victim of https://github.com/doitsujin/dxvk/issues/1245 . That bug has been resolved in the steamvr client, so I asked @doitsujin to confirm if this game was blacklisted due to that bug (in which case that could be reverted) or for another reason.

Well, that was... Interesting to test. I downloaded dxvk 1.5.4, a release a few days earlier than the DXVK patch that disabled the openvr DLL in Subnautica ( doitsujin/dxvk@07fee4a ), and installed it via setup_dxvk.sh in subnautica's pfx path, and changed proton back to 5.13-1. The game started and was oriented by the Valve Index headset, BUT it wouldn't appear in the headset, only in the regular PC screen!

Creating a dxvk.conf file in the game directory with dxvk.enableOpenVR = True works for me.