There are all kinds of art. There are some kinds of art that are quite hideous and unprofessional looking, like some abstract art. So if splashes of paint count as art, then so can a carefully designed vintage ad. I would rather have the latter on the wall in my home.
I think it is more artistic than some of the "modern art" that is sold these days. Paint splatters, random crap lines just smashed onto a paper, there is even a girl who pukes up paint onto paper and she calls THAT art. That is just ridiculous, and super gross.
Yes, I believe vintage advertising can count as art because art is very subjective. Art is not something your can rigidly define and it is something that can clearly be identified as not art or art. This is what is great about art. It's a combination of the artists vision and the viewers perception.
Art is a finicky medium where people can suggest that anything could be art. Perhaps that is why some people insist that vintage advertising is art. Technically, it is not. Instead it is advertising and reflects a world where people were influenced in a different way. Why the push for vintage ads to be art? Well, companies would make millions on licensing fees if they could convince the public to put up their posters in the living rooms across America.