Totally anecdotal, but I started taking vitamin D supplements at the suggestion of my doctor because of my family history of colon cancer. I noticed that my acne declined significantly. I'm wondering if it has to do with the amount of time one spends outside in the sun.
I started Vitamin D supplements too, after reading several personal accounts of it being helpful. Still have not come across any good studies proving the effectiveness of taking Vitamin D supplements (TBH I also haven't tried finding any such information either).
Snake Oil Supplements?
Scientific evidence for popular health supplements
Showing tangible human health benefits when taken orally by an adult with a healthy diet (Mar 23rd 2017)
"Vitamin D supplementation decreases all-cause mortality in adults and older individuals" http://acpjc.acponline.org/Content/pdf/ACPJC-2008-148-2-030....
(It's a review of randomized control trials of Vitamin D supplementation.)