Metabolism. B vitamins are critical components of cellular metabolism. They are catalysts, or assistants, for most of the reactions that turn food into energy in the body. If you do not have enough b vitamins, you do not turn food into energy in an efficient manner. This leads to low energy levels, low blood counts, etc.
Not the cause. Vitamin deficiency is extremely rare in the U.S. It is highly unlikely that fatigue is related to any vitamin d deficiency in this country.