first one is also an anecdotal evidence since there is no official vegan opinion, it's a personal choice on what to consume not some kind of cult with leaders.
the second one I don't find bizarre, if you don't want to consume animals/their products based on ethical choice, it's only logical you want to take care of your health by taking supplements. if you're arguing it's super-healthy and at the same time taking supplements then it's bizarre, yes.
third one probably depends on how flexible said people are
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In my personal experience as a vegetarian and seriously considering veganism most looking down is happening in the opposite direction actually, it's a lot more challenging socially to be a vegetarian/vegan than to be a meat-eater