...oookay, so I've got some progress. Thanks for tip juls2008 ! Turns out the sliders actually works, however in the driver view only, it doesn't work on foot (and I was standing outside the bus, while trying at first). I actually see the potential of playing this game in VR - that'd be amazing. The driver view is by default slanted towards the ground, plus there are some very heavy camera restrictions around turning your head around. With an FOV around 120, it's possible to drive the coach, and feelsrelatively natural, that's great!
If the FOV slider would control all-aspects of the game, not just the driver view, and the driver camera would be more permissive, and possible not centered at a downward angle, this would be a very much functional VR experience. Because it's not rendered in stereo, Vorpx VR performance is the same as in regular mode. Great progress!