In my view no matter what the tank originally was, it ceases to be a vehicle when it can no longer act as a conveyance. A tank (and any vehicle) is not just a form, but a function. Is a car with no motor, no tires, no transmission a vehicle? I do not believe so, because it cannot and lacks the possibility of moving without a substantial transformation.
As I said, if the thing can (i.e., without significant, non-trivial alterations to its current form) act as conveyance then it is a vehicle. It need not be in the act of conveyance, nor must it have an independent means of propulsion.
Edit: my simple rule breaks down when thinking about a flat piece of wood attached to rope. My mind says that's only a vehicle if someone chooses to use it as such for pulling objects or people. I'll leave the search for a universal maxim to others, it seems.