The problem isn't that you won't get enough sunlight if you lay down naked in the sun, the problem is that you can't because it is too cold.
Above/Below a certain latitude (N/S) there is a window of time where the sun never hits that angle at any point during the day, so you could sit outside naked all day long and still get zero vitamin D.
If this seems counterintuitive consider if you keep going far enough away from the equator and get into the arctic/antarctic the winter has no sunlight at all, and you clearly can't produce Vitamin D from sunlight if the sun never rises.
My city in Canada about 46.2 degrees north and about 300km from the eastern Maine border. Here in winter it gets light at 8am and dark at 4pm if there are no clouds. So most people are at work they go to work in the dark and leave when it is dark.
All the northernmost US states have a period of insufficient sunlight/angle to produce Vitamin D, and basically all of Canada is north of that.
Maybe there's an island that is Canadian territory that is south of say New York or San Francisco that I don't know about of course.