@ Smurf - I would agree with Sqad. Evidence for the efficacy of Vitamin D when it comes to Upper Respiratory Tract Infections is poor.
From a recent study, well controlled Randomised Controlled Trial carried out between 2010 and 2012 on 320 or so people
"In an accompanying editorial, Dr. Jeffrey A. Linder, M.D., MPH, an expert from Brigham and Women's Hospital and Harvard Medical School, wrote that the "results suggest that vitamin D should join the therapies listed in the Cochrane reviews as being ineffective for preventing or treating upper respiratory tract infections in healthy adults."
The only caveat is that for those people who are actually deficient in Vitamin D then supplements may possibly prove beneficial. All of the participants in the study, above, had normal levels of Vitamin D.
http://www.huffingtonpost.com/2012/10/02/vitamin-d-colds-upper-respiratory-tract-infection_n_1932922.html