B12 (also called cobalamin due to its central cobalt atom) is a water-soluble vitamin with a very low recommended daily intake requirement, about 2-3 micrograms per day. In addition to having extremely low intake requirements, Vitamin B12 is stored in the liver, kidneys, and muscle tissue, and most B12 (65-75%) is reabsorbed by the body instead of excreted. A deficiency could take from 5 to 20 years of inadequate intake to develop. There are many vegan foods fortified with B12. They include non-dairy milks, meat substitutes, breakfast cereals and some nutritional yeast. Sublingual vegan B12 supplements are also available. Although may nutritional experts agree that taking supplements is generally a bad practice, there is consensus that popping an occasional B12 couldn't hurt and may be beneficial. Although B12 is available in different forms, methylcobalamin is preferred over cyanocobalamin. Most B12 is derived from animal sources, however, it is possible to find vegan B12 at natural food stores such as Whole Foods.
Additional information about B12: