I have been a vegan on and off again in the past (for me, I don't care that much about the animals, but I view flesh as not being a terribly healthy food source).
As someone else mentioned, you need to take a vitamin B12 supplement. B12 in our diet only comes from animals. Unless you have a medical condition called pernicious anemia, where your immune system destroys the cells in your stomach that produce the protein necessary for absorbing B12 (called intrinsic factor), you should have no problem absorbing straight B12. Your body stores it, so even without a supplement it would take 1-3 years at least before your levels got low, possibly longer. Even so, supplementing is VERY important, because very low levels of B12 can lead to serious nerve damage (you would have other symptoms, like fatigue and anemia first). I would recommend telling your doctor about the diet change, too, since he/she might want to check your levels every year or two just to be safe.
That is the ONLY essential vitamin you can't get in a standard vegan diet. Frankly, vegetables are much better sources of just about every other vitamin (including all the other B vitamins) and most minerals. Veggies have less iron than meat, but you are male (no monthly bleeding) so your risk of developing iron deficiency is very low. The body has no way of excreting iron (it just recycles inside us) so supplementing is most likely not necessary (just eat your green leafy veggies like spinach, kale, etc.) If you aren't going to be getting your calcium and vitamin D in soy or rice milk, you should take a calcium supplement with vitamin D, too.
Really, a standard male multivitamin (no iron) that has 100% RDA of B12 should cover you. Or you could just chew a viactive calcium chew with vitamin D and take a daily B12 capsule.
Good luck with the veganism! I think I feel healthier when I don't eat meat, and I still eat little meat. I just try to buy from small farms that treat their animals well. Plus, once I tried soy cheese and yogurt (bleech!), I realized that full veganism just wasn't going to work for me. ;-) I need my triple-cream brie!
