Interesting to know. The minimum degree the sun needs to have for you to get Vitamin D is 30°. So as long as it's warm enough and the sun is high enough, you will get Vitamin D by tanning.
For example Belgium:
50° latitude North
4° longitude East
From 22 February onwards, the sun will go above 30° around 12 o'clock. So from then on, you can start getting out in the sun again to get your Vitamin D. From 15 April onwards the sun will go above 50°, which is the optimal time to get a lot of Vitamin D with great efficiency.