Some evidence indicates that Vitamin D intake may be a bad idea for those who have chronic autoimmunity conditions.
It is possible that some autoimmune diseases can cause vitamin D hypersensitivity. In these cases, it can actually suppress the immune system, causing hormonal disruption and heightened bacterial activity.
The transcript of a talk on the subject, linked below, offers an interesting counterargument to the idea, held my many, that vitamin D is beneficial for autoimmune disorders. More study is likely called for to determine the answer.