As I said, we have had this discussion before. The so-called vitamin D scam that Marshall heavily publicizes just happens to be essential to the promotion of his protocol and his financial interests.
It is misleading to call Vitamin D3 immuno-suppressive when it is actually immuno-modulatory. Vitamin D3 is a natural substance and the vast majority of studies indicate that supplementation can be beneficial for the vast majority of people. Studies also indicate that the majority of people are deficient in Vitamin D3. They also indicate that the very large majority of people are also deficient in magnesium, which is another natural item that is immuno-modulatory. Thus, for those people who are unable to get adequate sunshine and who are unable to get enough magnesium in their diets, I very much favor supplementing those items, in reasonable amounts, the same as I do any other nutritional deficiency which cannot be addressed by diet (which should be the first option, granted). Better that than having an unnaturally suppressed immune system or an unmodulated immune system which can turn itself on the body with devastating consequences.