It seems to rate pretty high (high=credible) on this list, which I often consult because it is easy to understand and easy to click-through to the actual sources: http://informationisbeautiful.net/visualizations/snake-oil-s...

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.)