If you want to split hairs, you should do it properly
In some regions there is not enough sunlight to produce enough Vitamin D. So it will be substituted.
https://en.wikipedia.org/wiki/Vitamin_D:
"Rickets was formerly a major public health problem among the US population; in
Denver, where ultraviolet rays are about 20% stronger than at sea level on the same latitude,
[39] almost two-thirds of 500 children had mild rickets in the late 1920s.
[40] An increase in the proportion of animal protein
[38][41] in the 20th century American diet coupled with increased consumption of milk
[42][43] fortified with relatively small quantities of vitamin D coincided with a dramatic decline in the number of rickets cases.
[1] Also, in the United States and Canada, vitamin D-fortified milk, infant vitamin supplements, and vitamin supplements have helped to eradicate the majority of cases of rickets for children with fat malabsorption conditions.
[27]"
In regards to the Inuit and Eskimo their diet of whale, seal, and walrus blubber (vitamin D saturated fat), along with eggs and char (trout) are all rich in vitamin D. These northern peoples did not rely on the sun for vitamin D, they consumed it.
So wouldn´t it be right to say that in part of the world Vitamin D can´t be produced in sufficient quantities by the body itself and therefor is in these parts of the world a Vitamin?