Humans could survive quite easily on a non-animal diet
No, they couldn't. Without the help of modern medicine and industry, providing self-inflicted-malnutritionists (I hope I got that one right) with most of the meat-substitutes they need, vegeterians and most certainly vegans wouldn't be able to survive for very long.

Vegeterians don't eat meat, so they eat more of other things. They get enough calcium, protein, vitamins and even iron (with the help of modern food industry). They wouldn't be very active or producive in a wild setting, but they'd live, if they have the physical protection they need from the stronger, all-eating-fully-nutritioned humans.

Vegans however, not eating meat or dairy products, would perish very quickly in a rural setting. Without tofu, soya and artificial vitamins ( a very pricey and difficult to manufacture product, by the way, and one of the reasons the only living vegans are from a high socio-economical setting), vegans wouldn't live for very long.

It's quite simple, really - modern society has many illnesses... it mistreats humans and animals alike. Vegans want to change that... however, they fail to see that without the help of the same modern setting and enviorment they live in, their deaths would be inevitable. "Don't kill the cute animals, but please keep on producing all these wonderful vitamins that I rely on to live!"