I’ve overcome the issue – rename the Steam VR folder (eg by adding “a” to its name) and thereby fool the system that there’s no Steam VR – launch FBVR with the headset software open (eg Oculus) and it works.

    One obviously needs to rename the Steam VR folder back to its original name if one then needs Steam VR to play another game.

    (7) How to disable SteamVR from launching – YouTube