[jlcgull]

As a corollary to this, I am also curious about vitamin D levels in people using UV tanning beds.

Assuming equal carcinogenic potential from sun exposure and from such beds (which is most likely an incorrect assumption), the results should be insightful.

[carbocation]

So long as they're getting exposed to UVB, they should be producing Vitamin D. E.g., http://www.nature.com/jid/journal/v125/n5/full/5603599a.html

The absolute amount of Vitamin D needed is quite modest, and the risk of cancer is (typically seen as) proportional to sun exposure; hence the recommendation for some - but not much - UV exposure.