In breast cancer proper researchers have seen that there is a clear link between lower level of vitamin D and aggressive disease. Most commonly associated is the triple negative breast cancer; those are negative for estrogen receptor (ER), progesterone receptor (PR) and epidermal growth factor (HER2).
There is less targeted approach to combat this cancer in form of neo-adjuvant therapy; those are possible with breast cancers positive for receptors. Metastasis is more often seen in triple negative breast cancers.
There is no clear cut opinion in the medical fraternity whether to supplement it or not. But, it sounds reasonable to have normal level either naturally or from supplementation.