Adults need to have 2/3 of their skin exposed for 20 minutes to make their daily need for Vit D. DS and I are so pale we're basically see-through, that much skin for that long is just a recipe for sunburn.
FWIW, I'm in the camp that thinks that Vit D is a miracle substance. It's not exactly a vitamin, it's a precursor to hormones, and all of its roles in the body are still not yet known. Personally, I struggled for years with infertility. I did a TON of reading about everything in the world that could be wrong with us, and one study showed that rats with Vit D deficiency had smaller litters and higher rates of infant mortality. Some theorize that Vit D may contribute to infertility and miscarriage. I got a blood test and was severely deficient. After 6 weeks of high-dose supplementation, I got pregnant on my first full cycle with normal Vit D levels, and carried this perfect little guy to term. It's anecdotal, of course, but I'm a big believer.