Total percentages of vegetarian and vegan Americans do appear to be quite low, and they skew more liberal and lower income[1]. I'm not the person you're responding to, and it requires some extrapolation, but the liberal skew does at least suggest that vegetarians may skew "metropolitan" as well.
I appreciate what you're trying to do here, but "skew metropolitan"(your point) and "aren't common outside of certain metro areas"(op point) don't feel like the same rhetorical weight.
I mean my thoughts are the whole "Vermont Hippy" seems like a thing, and IDK that Vermont even has a meto area (/s).
I don't have a dog in this fight, but I did run into this (which doesn't answer the question, but is interesting)