Vegans need B12 supplementation.
Maybe take a look at this documentary, which touches on the story of a vegetarian who ended up living with the Inuit and his experiences. It's a good place to start to expand your knowledge.
https://www.youtube.com/watch?v=iVJM_0XEiBI&pp=ygUPZmF0IGRvY...
Conversely, there has never been a vegan society. Even the handful of vegetarian cultures that have evolved such as those found in India rely heavily on dairy. Humans simply aren't adapted to be vegetarian.
The problem with meat is that it doesn't contain carbohydrates, which are a cheap source of energy, so the body has to work a lot more to get its energy, and with a lot more by-products. Also, the lack of fiber will significantly alter the gut microbiome, and almost always leads to stool issues.