A couple great posts about vitamin D posted to HN:

-https://news.ycombinator.com/item?id=24138590

-https://news.ycombinator.com/item?id=24261124

-And her part 3 (which links to tons of studies and is quite informative): https://www.devaboone.com/post/vitamin-d-part-3-the-evidence

My TLDR from reading those posts: Vitamin D overdosing is possible, can have some pretty severe consequences.

Vitamin D deficiency is correlated with tons of conditions. But this seems likely to be a correlation and not a causation. Simply because unhealthy people spend more time indoors than healthy people.

The relevant statement in her post from part 3:

"Vitamin D seems to reduce asthma attacks in adults with mild to moderate disease, and daily or weekly Vitamin D seems to prevent acute respiratory infection in those with Vitamin D less than 10 ng/ml (25 nmol/l). There is considerable excitement around Vitamin D's potential role in Covid treatment, though we do not yet have enough evidence to make a definitive conclusion"

There was a good discussion a ~16 months ago (and the author even showed-up to comment) on an MD's article re: vitamin D that discusses toxicity: https://news.ycombinator.com/item?id=24138590