RPM
The Archdiocese of Detroit
http://www.aodonline.org/AODOnline/History+and+Archives+1243...may be able to direct you to the proper location of any documents. They are not in their archives, and state they are the responsibility of the actual Charity running the orphanage. In the case of St. Vincent's it is the Sister's of Charity.
http://freepages.genealogy.rootsweb.ancestry.com/~detroitchu... I was able to find what is possibly my great grandmother in the MI census of 1860 living in St Vincent.
Have you tried the 1930 census on ancestry.com
Do you have any more details...ie, name, year born, possible parish baptized, if Catholic. Heading to the Burton Library next week in Detroit to research. I can keep it in mind in my searches of Catholic parishes etc.