Some people are just rude, and have to pass comment on things - not just dietary choices, but many things in life.
I don't know many vegans, but the two I do are atrocious. One posts things on Facebook all day every day about why we too should be vegan, and videos of animal abuse which I don't even know how they're allowed on there. If I get tagged in a photo (NEVER post) in a restaurant with a plate in front of me she will pass comment telling me what I've killed, and whose life I'm making a misery with my choices.
The other is my brother in law, who insists on lecturing FIL about how he would NEVER have had his heart attack if he had been vegan. He has bought him a lengthy book about the benefits of veganism and quizzes him about it when he visits, plus cooks him vegan dishes and takes them round. That's perfectly nice, possibly, but FIL and MIL are absolutely independent and cook for themselves daily, and are not choosing to be vegan, so I think it's rude as hell for BIL to be making comments.
I had my birthday meal in a curry house, and he asked whether we could, instead, go to a local vegan restaurant so he would be catered for. I said no, as there was a decent vegan range at the curry house (in fact I have their vegan vegetable curry often as it's really nice) and he declined to come because they cooked meat. I imagine his choices for eating out are rather limited.
So. On the whole, vegans/vegetarians are fine. I was vegetarian myself for 4 years, and tbh people didn't really know - because I didn't feel the need to announce it. If I was out, I ate a veggie dish, no dramas. No comments.
But the ones who are pushy ruin it for everyone else, sadly.