Why are you making all this fuss?
Sure, there isn't a solarium on ISS right now, but it seems to me that if there were one, this Vitamin D thing would be a non-issue. Right now, it's cheaper to fly vitamin supplements than a whole solarium. In the future it may be vice versa.
Would you like to list ALL the perceived nutritional problems there are with space travel, then we can make a rational assessment of the "problem"...