[Barrin92]

is this just an extrapolation or does my body actually constantly produce the same amount of vitamin D if i sit in the sun even if I don't have a deficit?

Because usually phisiological processes are in some kind of equilibrium and regulated, so it's not straightforward to compare it to supplements.

[trevorkoob]

7-dehydrocholesterol is photolyzed by UV to previtamin D. There is then spontaneous isomerization between this previtamin D, vitamin D and 2 inactive isomers. As vitamin D is used by the body, and the inactive isomers remain in the skin, there is an equilibrium - as vitamin D is used by the body it is drawn from the skin, leading to more conversion of inactive isomers to active vitamin D.

[disgruntledphd2]

The article goes into some detail on this, my understanding is that Vitamin D is produced in inactive form with sunlight exposure and then this is more-slowly released as needed (over winter apparently, but nobody's quite sure apparently).