To get the game to launch in VR, we need to add back some of the Unity VR code. There's a way to do this without touching the game code, but this method that I'll temporarily explain requires adding a couple lines to the game's config file.
Run this exe, locate your game folder and install it.
This will tell Unity that it's a VR game. You can see how to manually do this process here:
How To Force Any Unity Game to Run In Native VR Mode
Copy these VR dlls into your game's plugin folder. They are the basic VR DLL's Unity adds automatically when you compile any VR game.
Should be somewhere like C:\\Program Files (x86)\\Steam\\steamapps\\common\\SubnauticaZero\\SubnauticaZero_Data\\Plugins\\x86_64
That's it! Now try running your game. You may need to force it to load in VR by adding launch options such as -vrmode Oculus or -vrmode OpenVR in the Steam or shortcut launch params
Here's a guide on what else needs to be fixed with modding to make it work.