When it's a hormone.
That's the stand that several researchers in vitamin D have taken and I think they're right. Dr. John Cannell has made a fuss over this in his www.vitamindcouncil.com website.
Structurally, vitamin D is most closely related to testosterone, estrogen, and cortisol. You wouldn't call testosterone vitamin T, would you?
Vitamins are also meant to be obtained from food. Yes, vitamin D is in milk but only because humans are required to put it there to prevent childhood rickets. Otherwise, the only substantial food source of vitamin D is in oily fish like salmon and then only a modest quantity.
Vitamin D is cholecalciferol, a hormone. Deficiencies of hormones can have catastrophic consequences. Imagine that every winter your thyroid gland shuts down and produced no thyroid hormone. You'd get very ill, gain 30 lbs, lose your hair, feel awful.
That's what happens when you're sun deprived and thereby deficient in cholecalciferol--you're deficient in a hormone. And it happens to most of us every year for many months.
I continue to witness spectacular effects by bringing 25-OH-vitamin D3 blood levels to 50 ng/ml with supplementation, including an apparent surge in success dropping heart scan scores.