I'm neither a participant nor scientist in the study, so I don't see what point I'm missing. Actually even for both those groups, making predictions is fine, really, so long as the study is properly blinded.
From anecdotes, the correlation studies, and my personal experience, I believe the results will be positive. If the results show otherwise, I'll accept it and try to understand why I was wrong.
Late reply: I'm curious to see the results, but scientifically speaking Vitamin D is a tricky substance.
Determining if you have 'the right levels' is tricky, considering that the same sample can lead to 20-33% false positive rate of being misclassified as Vitamin D deficient[1], despite being measured using the same method, in two independent laboratories, compared with a different, gold standard: mass spectrometry.
Moreover, even in cases with low Vitamin D, where medical professionals are unsure if treatment with Vitamin D would be beneficial at all (in this case: for osteoporosis prevention)[2].
Finally, recent randomised, double-blind clinical trial (n=5108) assessed if monthly administration of a high dosage (100.000 IU) of Vitamin D for more than a year would in any way prevent cardiovascular disease, and found no benefit in patients receiving treatment or placebo after a 3.3 year follow-up period[3]. Another study (n=2303, 4-year long, 2000 IU/d), testing the efficacy of Vitamin D to prevent cancer, actually suggests that low dose of Vitamin D is may increase cancer.[4]
Taken together, I hope that this helps illustrate how 'making a prediction' or 'expecting a certain result' is neither scientifically ethical, nor possible, given the controversial results in the literature so far.