It's hard to imagine a reason why someone is vegan, that wouldn't logically entail such a self-image.
> they think the atheists think that they are superior (whether or not they really do).
Atheists do not present atheism as an abstention from religion, and generally describe religion as irrational or illogical, not immoral.
Of course everyone who consciously makes a socially difficult choice has a well thought-out reason for it (although atheism is not particularly socially difficult in my environment). But they aren't all directly comparable.
> It's hard to imagine a reason why someone is vegan, that wouldn't logically entail such a self-image.
I know a guy that’s vegan because he shit himself after going to an Outback Steakhouse
As for "feasibility", that is clearly not the same experience for everyone.