This is pure speculation but...
I suspect that vitamin D is most valuable as a preventive agent. That is, to keep the body running properly so to not develop debilitating conditions. OTOH, maybe some conditions can be reversed with vitamin D supplements.
Or a nice double blind multidecade prospective study.
It would be easy to set up a study in Saudi Arabia linking low vitamin D levels to breast growth. As soon as their breasts grow, their vitamin D levels drop. Ignoring the fact that after menarche, Saudi women are cloistered would add as much bias as ignoring that sick people tend not to go outside as much.