Steam Deck runs an embedded session of Gamescope which uses DRM (no not copy protection) to display games.
Running Gamescope as a launch command will run it in a nested session, where it’s output is being sent to your display manager, ex. Kwin. That’s where the lag can happen, and usually does especially in GPU bound scenarios
You can play the mod on any SteamVR compatible headset, including the very first HTC Vive, the Oculus Rift CV1 and many others. You don’t need an Index to play it