Steps to reproduce:

1. Disable Vehicle Bar
2. Get on Vehicle

Looks like MainMenuBarVehicleLeaveButton is attached to the MainMenuBar and since BT4 replaces that when you get on a vehicle the button is not displayed. Guessing need to detach it and add a mover the button also does not show up in Edit mode with BT4 enabled for the same reasons.

The Vehicle Bar's only purpose is to make the leave vehicle button movable. If you disable it, naturally it will not be present.

I will circle back with the person that was reporting it to me. Think there was a mixup between use blizzard vehicle ui and vehicle bar. I just did not catch it.