What are you talking about? Especially vitamin D is researched to death. You think scientists are stupid? If there is a incidence difference along the north-south axis, it's the first hypothesis investigated.
Thing is, intervention usually doesn't prove very effective. That's why doctors aren't that hysterical about it.
Then some vitamin supplement studies have shown actually harmful, eg. vitamin A as antioxidant. Vitamin D is a hormone and regulates many processes in the body, e.g. a large dose suppresses the immune system similar to steroids... caution is definitely warranted, instead of blanket recommendations of the "Just take 5000IU/d! #YOLO" you find online frequently.
I didn't say researchers, I said in a medical context. If you do a search on pubmed or Google Scholar, you can find a lot of research on how vitamin D supplementation proved beneficial for patients in different contexts.
Also, I found your reply really jarring. Perhaps it's the immediate ad-hominem attack, but I just wanted to note that for your reference, it really didn't motivate me to want to engage any further with you.
Good thing I also mentioned doctors, who are, of course, applying the concluded result of research. But maybe you are talking about another "medical context".
I threw your "vitamin D supplementation proved beneficial" into Google and this Nature review article on vitamin D supplementation was the first result: