I have spoken to two doctors about the Vitamin D deficiency issue and they both told me that it is a very controversial issue in the medical field. One of them was a doctor (my doctor at the time) who herself was VERY into this idea that Frum people (men as well as women, since Frum men also tend to cover themselves pretty thoroughly) are lacking Vitamin D, but she herself acknowledged that it was a controversial position and that most of her colleagues disagree with her.
The other doctor I spoke to explained the reason why a lot of people nowadays are getting blood tests results that claim they are deficient in Vitamin D. He said that it’s because they have increased the amount that they decided that people need, and there was not necessarily a good reason for this. When I asked him about this idea that Frum people don’t get enough Vitamin D, he also thought that was ridiculous, and he said that you can get enough Vitamin D just by standing at the busstop for 15 minutes on your way to Kollel.
The interesting thing is that the first doctor is herself Frum and the second one isn’t. I know this isn’t a complete proof, but I personally find it hard to believe that Hashem would create a world in which it is necessary to violate Halacha in order to get the Vitamin D we need. That is why I am skeptical about the whole thing.