Wikipedia: Vitamin D
...and now for the kicker.
"Ironically, there are indications that vitamin D deficiency may lead to skin cancer." 
(Am I the only one laughing here?)
"To avoid vitamin D deficiency dermatologists recommend supplementation along with sunscreen use."
Here's the problem with the recommended supplementation of vitamin D, whether it be from cod liver oil or a vitamin bottle:
'Sunshine vitamin' may ward off breast cancer
Higher levels of vitamin D offer range of health benefits, studies suggest
msnbc.com news services
updated 6:48 p.m. ET, Thurs., May. 11, 2006
"Women who get lots of vitamin D are less likely to develop breast cancer, suggests a pair of studies that add to the already strong evidence that the “sunshine vitamin” helps prevent many types of cancer."
"High levels of vitamin D translated to a 50 percent lower risk of breast cancer, one study found. Even modestly higher levels resulted in 10 percent less risk, which would translate to 20,000 fewer cases a year if it were true of all American women."
"A second study, by Canadian researchers, found that women who spent time outdoors or got a lot of vitamin D from their diets or supplements — especially as teens — were 25 percent to 45 percent less likely to develop breast cancer than women with less of the nutrient."
Heliotherary (sun-therapy) for tuberculosis patients:
By Dr. James Howenstine, MD
"Research from UCLA  and the Harvard School of Public Health revealed that vitamin D plays a key role in the production of a molecule called cathelicidin, which kills the mycobacterium tuberculosis organism."