For a healthy dose of skepticism, check out the New York Times article "Vitamin D, the Sunshine Supplement, Has Shadowy Money Behind It". It points out, among other things, that a 2010 National Academy of Medicine (then known as the Institute of Medicine) report found that most Americans get enough vitamin D, and that "the evidence supports a role for vitamin D and calcium in bone health but not in other health conditions"
http://www.nationalacademies.org/hmd/Reports/2010/Dietary-Re...https://www.nytimes.com/2018/08/18/business/vitamin-d-michae...
On a related note about sunshine, Sam Altman recommends a full spectrum LED light in the morning as his most beneficial productivity booster.
http://blog.samaltman.com/productivity