Not sure of the specifics in Michigan. In some states it is against the law to wear body armor while committing a crime. So if your firearm use was determined to not be in self defense you could be facing extra penalties. But your pretty screwed in that situation with or without the vest.
As @Mr.Plastic points out a prosecutor might paint you in an unfavorable light for wearing one. I personally find this argument to be ridiculous though. A bullet proof vest is a purely defensive device. Why shouldn’t anyone who cares about their own safety be able to wear one if they are willing to put up with the discomfort? When they make one that is as comfortable as a Tshirt I’ll be first in line to pick up one for everyone in my family. At the moment the discomfort is not worth putting up with given the very slight chance I would ever need it.