I wouldn't expect the clamp version to ever come off, however I could see it shifting under bad vibration, which is likely why
Trillium told me to add some fasteners to it. I'm betting they had to fix one before.
If you're worried about head room, make sure you buy the high profile vent to start with. It sticks out of the roof much more than the low profile vents. Then if you need to make a spacer it won't be as thick or you might not even need one.