There is no civil union in Michigan so there would be no need for an annulment in Michigan because it wouldn't be recognized in Michigan as a marriage.
You should talk to an attorney if you have any other questions. Good luck to you.
You've asked a very interesting question. The previous poster is correct, Michigan does not recognize civil unions. However, I would add that Michigan does recognize out of state judgments, and if there is a judgment in Vermont for support, you may be required to pay any monies on that. Assuming that both parties reside in Michigan, this may or may not be an issue, however I would contact a Vermont family law attorney and pose your question to them.
Good luck and thanks for posting.
The information provided is based solely on the general information given and should not be construed as legal advice for your specific situation.