Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production. –wikipedia

Ok, so that sounds important. Why do vegans need to pay attention to this?

Vitamin B12 is naturally found in meat (especially liver and shellfish), milk and eggs. Animals, in turn, must obtain it directly or indirectly from bacteria, and these bacteria may inhabit a section of the gut which is posterior to the section where B12 is absorbed. Thus, herbivorous animals must either obtain B12 from bacteria in their rumens, or (if fermenting plant material in the hindgut) by reingestion of cecotrope fæces. Eggs are often mentioned as a good B12 source, but they also contain a factor that blocks absorption.^[27] Certain insects such as termites contain B12 produced by their gut bacteria, in a manner analogous to ruminant animals.^[28] An NIH Fact Sheet lists a variety of food sources of vitamin B12.

According to the U.K. Vegan Society, the present consensus is that any B12 present in plant foods is likely to be unavailable to humans and so these foods should not be relied upon as safe sources, as the B12 analogues can compete with B12 and inhibit metabolism. Also, vegan humans who eat only plant based foods must ordinarily take special care to supplement their diets accordingly. The only reliable vegan sources of B12 are foods fortified with B12 (including some soy products and some breakfast cereals), and B12 supplements.^[29]

While lacto-ovo vegetarians usually get enough B12 through consuming dairy products, vitamin B12 may be found to be lacking in those practicing vegan diets who do not use multivitamin supplements or eat B12 fortified foods. Examples of fortified foods often consumed include fortified breakfast cereals, fortified soy-based products, and fortified energy bars. Claimed sources of B12 that have been shown through direct studies^[30] of vegans to be inadequate or unreliable include, laver (a seaweed), barley grass, and human gut bacteria. People on a vegan raw food diet are also susceptible to B12 deficiency if no supplementation is used^[31].

The Vegan Society, the Vegetarian Resource Group, and the Physicians Committee for Responsible Medicine, among others, recommend that vegans either consistently eat foods fortified with B12 or take a daily or weekly B12 supplement.^[29][43][44] Fortified breakfast cereals are a particularly valuable source of vitamin B12 for vegetarians and vegans. In addition, adults age 51 and older are recommended to consume B12 fortified food or supplements to meet the RDA, because they are a population at an increased risk of deficiency ^[45].

Ok, Now I see that as vegans we need to really monitor the intake of our B12 as it is not really found in our natural diet.  What are some recommendations?

Brewer’s and Nutritional Yeasts

Brewer’s and nutritional yeasts do not contain B12 unless they are fortified with it. At least two vegan B12-fortified yeasts are currently on the market: Red Star Vegetarian Support Formula and Twinlab Natural Nutritional Yeast (verified to be fortified with B12 via personal communication with Twinlab June 3, 2003). Unfortunately, there are some drawbacks to relying solely on B12-fortified nutritional yeast for B12:

• Nutritional yeast often comes from bins in health food stores. If not careful, it would be easy for a store employee to order the wrong nutritional yeast out of the distributor catalogs which often list many yeasts. It would also be easy to accidentally put the wrong yeast into the Vegetarian Support Formula bin.
• B12 is light sensitive. Nutritional yeast is likely to be exposed to the light because it is often stored in clear bins or plastic bags.
• At least one vegan who thought he was getting B12 from nutritional yeast developed B12 deficiency symptoms that cleared up upon taking a B12 supplement. — Vegansource

Personally I use nutritional yeast all of the time, but now I will look into vegan b12/bcomplex options.