There have been a lot of studies performed that show that vitamin D helps prevent cancer, but overall there are even more studies that show no effect.
Results by types of cancer
No effect on breast cancer.
No effect on colorectal cancer.
Mixed results for prostate cancer.
Other types of cancer
Vitamin D production in the skin seems to decrease the risk of several solid cancers.
Improving calcium and vitamin D nutritional status substantially reduces all-cancer risk in postmenopausal women.
Vitamin D and/or calcium supplementation also showed no overall effect on CVD, cancer, and mortality.
This meta analysis concluded "the effect estimate for vitamin D supplementation with or without calcium for… cancer (seven trials, 48 167 patients)… lay within the futility boundary, indicating that vitamin D supplementation does not alter the relative risk of any of these endpoints by 15% or more."
Molecular basis of the potential of vitamin D to prevent cancer.