There are 300 studies on Vitamin D and fertility in humans on PubMed (a research database). Its importance has been shown for everything from menstrual cycle abnormalities, egg/embryo quality, uterine receptivity and implantation, and likelihood of conception.
Vitamin D also has a very important role with respect to progesterone:
Calcitriol (a Vitamin D precursor) helps prepare the endometrium to be receptive for embryo implantation. Moreover, it supports the implantation process and the course of pregnancy through different but similar pathways to those used by progesterone, proving to be highly synergistic. It is increasingly evident that Vitamin D provides essential support from the luteal phase through to early pregnancy and beyond.