As a vegan or vegetarian it is very hard to supply yourself with enough B vitamins without taking a supplement or eating fortified foods, especially B12. This is the reason going 100% Vegan can become unhealthy. If you are taking a supplement and think you are 100% Vegan, you may want to make sure. While some companies are producing B12 in labs, they cannot say whether or not it is bio-available to us as humans. A lot of B12 supplements are derived from animal products. Despite this sad fact, making sure you have the B vitamins you need will allow your body to absorb all the other vitamins and minerals that are in your food. Without your B vitamins your body stops many necessary processes. Basically, without B vitamins your body has a very hard time doing anything and eventually a deficiency will lead to permanent disorders or diseases. So if you are planning to go vegan or vegetarian, please make sure you take your Vitamins! Love, Peace, and Candy Grease!
Check out this Wikipedia page on B vitamins...makes for interesting reading.