"Vitamin D" tends to only shift the TYPE of immune response, so you'd have to know what the hell you are doing and use combination therapy.
Secondly, you'd probably have to use the actual active form of vitamin D, not the prohormone. Does anybody round here actually use Vitamin D? (or does anyone still use Androstenediol to build muscle?). I know that it's a lot safer to use the prohormone.
The safely issue largely concerns calcium, which brings me to my next point. It would be best to use non-calcemic derivatives, and I wish that more of these would be offered as research chemicals.