I'm a vegan (who eats a balanced diet), and I feel healthy and happy. I just take a multivitamin and add Red Star Nutritional Yeast to various foods. It's tasty and chock full of B vitamins. I think the key is to eat a really varied diet with lots of fresh vegetables and whole grains. Vegans who eat nothing but pasta and junk food are very common. With some education and good recipes, you don't have to fall into that trap. Here is a great website:

http://www.organicathlete.org/