That is certainly the tricky part. With the Michael story coming to a conclusion in part III, with the death of Mary not himself, it is hard to imagine him having any more business dealings ever again. Legimate or otherwise.

The natural thing for Michael to do after that scene on the steps is simply live his many remaining years in miserable isolation, devoid of any life about him at all.

To have him as an advisor to Vincent just does not seem possible to me I'm afraid.