The frequency of light needed to make Vit D is in the UvB range, which is able to get to the surface only if the sun is more than halfway or so up in the sky.
UvB that makes Vitamin D is also the culprit in skin cancer and is what all the ozone hole hoopla is about.
You absolutly don't want a laser of that frequency hit you, for example have you ever overfried an egg and seen the dried up burnt black stuff that starts to form around the edges...
From Mercola.com
"Based on my study of the Atlas of Cancer Mortality for the United States [3] and the geographic variation of multiple sclerosis among U.S. veterans of WWII, and a reading of the literature, I think that 80 percent to 90 percent of Americans are vitamin D deficient."
"Those with the best vitamin D status live in Hawaii and rural regions of the southwest, which have the highest UVB radiation in July [4]. Those with the worst vitamin D status are those with darker skins and those who live in Alaska and urban regions of the northeast. Nearly all Americans are vitamin D deficient in winter/spring when there is not enough UVB reaching the surface."
http://www.mercola.com/2004/apr/3/vitamin_d_grant.htm
Living in south Texas I am well familiar with the effects of the sun.
My comment on 'laser' had to do with a new skin treatment using laser.