It's arguable whether Vitamin D is really a vitamin or a hormone, see
https://pubmed.ncbi.nlm.nih.gov/33549285/
Look at the molecular structure
https://en.wikipedia.org/wiki/Vitamin_D
that's a freakin' steroid with one of the bonds in the rings deleted
It's pretty well-established science now that vitamin D is a hormone, not a true vitamin. Vitamin D binds a nuclear receptor that regulates roughly 1,000 to 2,000 genes (5-10% of the human genome).
The "Vitamin D" moniker has just stuck around since it was named in 1922.
Yeah, it's just a holdover, the term "vitamin" originated from "vital amine", and vitamin D doesn't even have an amine group
Well neither does Vitamin C.
It is a cofactor though
https://en.wikipedia.org/wiki/Vitamin_C#Pharmacology