Some days ago I watched a minecraft video in wich using commands they made a stair wich was made of minecraft vanilla boats in their solid state in wich you can walk ON them so as also they were attached one to another and they are entities in game maybe something like that can be done with vehicles, adding as many boats with a deletted texture and wich dessapear when vehicle is descroyed.
Those use axis aligned bounding boxes. And that is an incredibly shitty (not to mention hopelessly inefficient) way to make vehicle solid seeing as vehicles aren't axis aligned. Even oriented bounding boxes (same thing but not bound to the world grid) would be a shitty way to do this as the shame of most vehicles would mean you could be blocked by blank space, which just looks bad.
Basically an entirely new collision system would need to be written, there is no system currently within minecraft that could handle this task properly. And unless you feel like implementing this:
it's not happening.