Don't confusing tanning and Vit-D. They are two different things. And you don't need to bake in the sun to get vit-D. In fact, the slower you get it- and the more spread out over all the body (hence nudity) the better since if all your exposure is concentrated on just your face and arms (as typical with the average clothed person) these areas get too much sun but can't produce enough Vit-D.
Smart nudists don't lay out in the sun- they get all sun they need while doing things like swimming and sports because with so much skin exposed no part has to be over-exposed. That's the irony of nudism- by exposing more skin- the body needs less sun to produced Vid-D. That way no part gets too-much.