Give it a couple decades. Most 'vegans' stop being vegan well before that. I'll let you Google why they do.
I gather this is a vague snarky comment loosely implying that vegans don't get enough B12. If so, it's a profoundly ignorant point that shows you don't have any knowledge of a typical vegan diet nowadays.
A couple of decades of not eating meat is a pretty significant improvement on you your carbon impact :)
I am not vegan, will never be, but I have known vegans for more than two decades and they are doing fine. We need to make it easy to make the right choice about food, and one way to do that is to stop making meat meat meat the ONLY cool thing. I am fine with vegans serving me tasty food, they make alternatives easier. If you do not want to be a full time vegan just switch to 3 out 7 days of the week or something like that.