First, take a look at this chart. Knowing the game engine for the game you want to mod should give you a rough idea of how difficult it will be to mod the game into VR

VR.jpg

Note: This chart will need to be updated soon though. UE4 will soon be about the level Unity is for modding with NewJavaman's amazing work

If the game you’re wanting to mod is a mono Unity game, then you’re in the absolute ideal position to create a VR mod. Here’s a very high level overview of the process

https://youtu.be/Gt_kIrmTl44

Here’s a bit more details on this initial process:

How To Force Any Unity Game to Run In Native VR Mode

Once you get your game launching in VR, it’s time to use a modding framework like BepInEx or MelonLoader to start fixing all the things that are broken.

Here’s a high level overview:

  1. Decompile the game’s code with a .Net decompiler like ILSpy
  2. Extract out the assets with uTiny Ripper and open that with the same version the game was built with
  3. Create a dummy project in Unity with the same version of Unity that the game was built with.
  4. Study all of the amazing existing Unity VR mods made to learn how they tackled certain things such as Subnautica, Valheim, Firewatch, Outer Wild, GTFO, Risk of Rain 2, Life is Strange, etc.
  5. There’s lots of guides and tutorials (videos and written) on modding Unity games with something like BepInEx. Doesn’t need to be VR specific, the same basics apply
  6. Familiarize yourself with normal Unity VR development. Once you decompile the game, it’s almost like you have the source code and are just coding from that. Many just normal “VR Dev work” will apply and there’s LOTs of videos and written tutorials online if you’re just getting started