The problem is in transtioning a player from being a walking player to being a controller of another object instead, and back.
The bugrigs physics can be used for vehicles (or other physics settings can be created), it's just that you need to beable to have a player who is set to normal physics approach a vehicle, "get in", have vehicle physics enabled, beabel to get out, have player model reappear, and vehicle be empty again.
Context switch.
The concept doesn't seem to complicated, though one needs to make a way to have selective physics settings per player etc.
BTW: The German Military is recruiting hackers now. Are they going to step up recruitment of real soliders next? And then build these vehicles we speak of?? Isn't "military" a banned word in Germany? If not, why not? I mean, we must be respectful of the past, and the past included military being bad.