Which VR headset works smoothly with Blender? I'm about to buy a VR headset. My choices are: Valve Index, Oculus Rift-S and Vive. Which one of them is 100% Blender compatible? I must say I'm leaning towards Valve. 

Thank-you very much !
Actually, SteamVR (which also means Valve Index) is now OpenXR compatible.
All you need to do is to select the current Beta of SteamVR (in Steam ..)
Works flawlessly with Blender - no other setup required.