Researchers in Australia are setting up a trial to see if Vitamin D can prevent the development of MS. They are going to give patients who are at high risk of developing MS who have had what they call a "clinically isolated syndrome", an attack of optic neuritis or numbness etc, different doses of vitamin D or a placebo. They are confident that their trial is well enough designed to show whether vit D is useful in preventing the onset of MS.

http://www.abc.net.au/news/2012-10-16/ms-trial-to-explore-vitamin-d-links/4314906

I hope that they give it long enough and follow up on patients, I can't imagine that any change is going to happen overnight or that they will know if MS has been prevented without a considerable amount of time having passed.

A couple of years ago I took part in a study in Scotland which was trying to ascertain the birth months of people with MS, the theory being that the foetus in the second trimestre of gestation might have not got enough vitamin D from their mother for proper development, if the second trimestre was in the winter months in a temperate zone. Interestingly they also asked for the birth months of parents and grandparents. My daughter was born at the end April,  and 5 out of 6 of her relevant family were born in March, April or May. I can't find any published results for the study as yet, but pasted below are a couple that looked purely at birth months.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC544426/

http://www.bmj.com/content/330/7483/120