Vitamin B12 comes on top of the Vegan Supplement Checklist, this because vegetable food does NOT contain any B12 (also not in tempeh, parsley, miso, seaweed etc.)
Our body can not make any B12 out of it's own.
Adults can have a B12 reserve of about 3000 micrograms, though an amount which can last for years (we loose about 0.05-0.2% of the storage daily) we should be aware that vegans do not restock themselves and vegetarians not enough. Depending on your lifestyle (smoking, use of medicines) your stock will be depleted in a longer or shorter period.

Vegans and Vegetarians should take a (daily) B12-supplement.

The only vegan sources of B12 are till here B12-pills, -drops or -injections, their source is that of bacteria. B12 producing bacteria are especially selected and factory cultured in huge boilers.
I use myself the vegan Solgar B12 pills, available in 100mcg, 1000mcg.

The US Recommended Dietary Allowances for B12:

0-5 months0.4
6-11 months0.5
1-3 years0.9
4-8 years1.2
9-13 years1.8
14 years+2.4

The three most important consequences that can occur by a lack of B12 are:
1 braked cell division, which can lead to; anemia, impaired functioning of the intestines, resulting in lack of appetite and weight loss. Further, there is an increased risk of inflammation in esophagus, mouth, tongue and urine leader.
2 damage to the nervous system. This may, among other express as a tingling sensation in the ends of arms and legs. It may also lead to insensitivity to touch, temperature, pain and impaired balance and coordination, and even in severe cases paralysis. In addition vision can deteriorate.
3 mental disorders, including depression, fatigue, indifference, anxiety, reduced concentration, psychosis, forgetfulness, confusion and irritability. The voice may become confused.

If you want to read more about B12;
What every Vegan should know about B12 from VeganOutreach 

Vegetarian Society; B12 Information Sheet
For Dutch readers you can downlowd a book of Michel Post; Vitamine B12 en Veganisme, een literatuurstudie.