I just tell them they're angry and delusional because the lack of Vitamin B12 in their diets is messing with their brain function.
I've gotten into on-line arguments with vegans concerning the lack of B12 in vegan diets to wit: if veganism is healthy, why don't vegan diets have a much needed vitamin in them? Why do vegans have to add B12 to their diets in the form of pills?
I either get no response or some sort of dodging of the question.
Another question I ask is if humans evolved as vegetarians/vegans, why do we have incisors which are used by mammals for ripping and tearing flesh?
Again, many times no response or evasion of the question.
I certainly don't care if people want to go on a vegetarian or vegan diet. That is their personal choice. But just don't try to tell me veganism is natural or that humans (and many other mammals) didn't evolve to be omnivores: meat and vegetables.