[obligatory insult]
Vegetarians are fucking retarded. Vegans doubly so. Enjoy your ethical fallacies, you pseudo-hippie wannabe.
[/obligatory insult]
A carefully managed vegetarian diet can indeed deliver everything your body needs, but vegans of course have it a bit harder. There are actually athletes and such that are vegetarian, but they tend to work very closely with professional doctors and nutritionists on a very strict diet anyway. If you're doing serious manual labor and you want to remain a vegetarian, you really should be consulting with a professional nutritionist (and not some bullshit New Age quack, I mean someone with an actual medical degree from a real medical school).
http://en.wikipedia.org/wiki/Vegetarianism#Health_benefits_and_concerns
http://en.wikipedia.org/wiki/Veganism#Nutritional_concerns
Oh, and the widely purported health benefits are taken WAY the fuck out of context. People have an evolved tendency to overeat whenever food is available and most people don't watch their diet very well. By far the biggest benefit of vegetarianism is quite simply that vegetarians and vegans have to watch what they eat much closer than normal people. They end up eating the right amount of food, but they also have to be careful to fully compensate for depriving themselves of major sources of things our bodies require.