"Vitamin D deficiency is linked to cancer, heart disease, respiratory infection, stroke, diabetes, and death.
It might seem reasonable that we should all be taking this supplement. But hold on. The argument for taking Vitamin D assumes a few things. First, that low Vitamin D actually causes all of the conditions it is associated with. This is not necessarily true. Low Vitamin D could cause (or contribute to) heart disease, but it could also be that heart disease causes low Vitamin D. Another possibility is that some third factor causes both low Vitamin D and heart disease."
https://www.devaboone.com/post/vitamin-d-part-3-the-evidence