Nephew "A" has PC parents and a food nazi mother who hates meat and butter and sugar; they are not vegan but have little in the way of meat and no dessert. PC Mom and Dad are both quite overweight and both the kids are short for their age and not so quick.
Nephew "B" has a Mom who feeds him little bits of food when they go out for a walk, and made sure from an early age that he had some meat every day. Mom and Dad are both in normal weight range and "B" is much stronger, tougher and faster physically and much sharper mentally than his cousin of the same age, "A".
Actually the Vegan diet is not better than other diets. Studies show they do not live longer than other groups e.g. meat eaters at all. Studies show overall that they develop quite a few group specific problems over the coarse of their lives more regularly and frequently than meat eaters.
Generally pride is the culprit behind a lot of the boasting that goes on with Vegans. People in general because of the fallen nature like to find things to distinguish themselves from the crowd (like being vegan or some other goofy fad) and then from that position of immagined superiority go to war with the crowd to gain converts.