adjl posted... "My primary *object*" indicates ownership of the object. "My primary *decision*" indicates ownership of the decision. "Vehicle" is an object (or class of objects), so if you talk about your primary vehicle, you are talking about your vehicle (note that the adjective can be removed without changing the fundamental semantic structure of the phrase). "Mode of transportation" is a decision, so if you talk about your primary mode of transportation, you're talking about your decision about how to get around.
Mostly, your problem is that you're using the word "vehicle" when you actually mean "mode of transportation." You don't own the vehicle. You do own the decision to use that vehicle to get around. "My primary mode of transportation is my car" is valid for communicating both the ownership of the car and the decision to use it to get around. "My primary vehicle is my car" carries the hidden third semantic piece of implying the existence of more vehicles that you own, then identifying your car as the one you use most often.
Basically, "my primary vehicle is somebody else's bus" doesn't really work at any level (except vaguely implying that you're a bus thief, I guess). Whatever logical gymnastics you manage to pull off in trying to pretend that it does work, it still doesn't work at all for attacking the sense of self-worth that anyone has attached to the value of a car that they own. "You're going to feel like I totally burned you if you interpret this awkwardly worded sentence in exactly the non-standard way that I want you to interpret it" doesn't exactly make for the most robust insult.
imagine actually typing this all out and posting it