There are six main vitamin groups that the human body needs for proper functioning. These are A, B, C, D, E and K. Out of these, A, D, E and K are fat-soluble, and are stored by the body to some extent in its fat reserves.

B and C, however, are water soluble and are excreted away as urine, when taken in excess. So these have more possibilities of being in deficit than the others.
Vitamin B is a family of substances grouped under the letter B that cause a number of significant body processes to go at a better pace and in a more orderly manner.
Out of these, Vitamin B12, also called cobalamin, is a particularly crucial cofactor in DNA production and fatty and amino acid metabolism.

The Wikipedia page on this particular substance besides listing it as the most structurally complex protein also states in quite the bone-chilling manner:
“Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system […] especially in elderly people (over age 60) who produce less stomach acid as they age, thereby increasing their probability of B12 deficiencies. Vitamin B12 deficiency can also cause symptoms of mania and psychosis.”
So, you better keep this particular vitamin in mind when planning a diet.
Here are some of the red-light signs of a deficit of this particular vitamin.