I get my protein, since I don't eat meat, from a plant based organic Protein Blend that gives me 25 grams of protein with each drink I prepare. Of course nuts, beans etc are good but I have terrible eating habits so that protein drink really helps. Made by Sun Warrior with all essential Amino Acids as well. available in most Health Food stores. Comes in Chocolate and Vanilla
There are other sources for B12 too, though, I believe. Kefir alone does the trick for me (I'm vegetarian and don't take any supplements whatsoever), but as it's a type of yoghurt I can't suggest it for vegans. Some mushrooms can apparently help.
Kefir is a cow product as is yoghurt. This question is regarding the vegan diet. B12 is found on the ground and it has been found on mushrooms but mushroom is not a consistent source of the vitamin.