That is a pretty good idea. We could just be arbitrary and say anything made before 1925 to 1930 or so is vintage. That could include everything: open cars, roadsters, covertibles, saloons, limousines, etc. Most everything that I am aware of after about 1930 or so fits in one of the existing classes.

I have no problem with being arbitrary since, in my opinion, the classes are a little arbitrary anyway, ground conveyer seems to be for anything that can't be placed anywhere else, pickup trucks in cars, chasis cab (whatever that is), etc.