More than you ever wanted to know about Vitamin D . . .
You may opt to follow the AAP guidelines for Vitamin D supplementation (you can read the research behind the policy here), which suggest that newborns should begin Vitamin D supplementation within the first few days of life.
If you would like to explore increasing the level of Vitamin D in your breastmilk through supplementation of the mother, you can learn more here.
Preterm babies have their own Vitamin D guidelines, which can be found here.