I know your text is about vegans. I myself am a vegetarian though, not eating things for which animals need to die. I do eat animal products like eggs and yoghurt and still drink milk. So maybe I am not the best example, but speaking as a vegetarian, I can at least say that after 10 years of practicing this lifestyle, I still feel great and energetic.
I think it's just a matter of a balanced diet, with some added help in the form of B12 vitamin supplements. Meat can be replaced with all sorts of things, like beans/mushrooms/nuts/lupine/seitan.
But I would really be interested in a long term study that would be able to show the difference between a carnivorous lifestyle and a vergetarian one. The pro's and con's of both. Not that it would matter much, because like you said, I made the change because of principles, it wasn't a health based choice.