I read people with uctd who have low vitamin d levels more likely to progress to defined disease. do supplements reduce chances of uctd progressing?

Why do you ask?: This question can't be answered, and anything you read online is at best "pop" impressions. Nobody even has criteria for "undifferentiated connective tissue disease" that everybody agrees on. If you're vitamin D deficient, and a great many people are, for everybody's sake take a sensible supplement. This common deficiency accounts for much of the bone pain that folks may think is CTD.