Physicians might know. Quacks don’t. If you simply google for vitamin D, you find so many quack websites as the top result. So your idea of keeping it a supplement is a bit questionable, if it is actually medication and is easily dismissed as harmless by quacks.
It is the responsibility of physicians to discuss diet and supplementation with patients, and know that food and supplements have potent acute and chronic effects, and these effects may be on par with prescription medications.