the whole point of evolving light skin was to get the necessary beneficial effects of sunlight (including vitamin D)

I’m not sure that we actually know that this is true. It could be that there is some other beneficial mutation which causes light skin which is only a net positive at higher latitudes where light skin is not penalized as much. And there’s some disconfirming evidence for the vitamin D hypothesis: people of African ancestry seem to do okay when they live at higher latitudes and most of us spend most of our time indoors, anyway, and also seem to do okay.