VMM stand for Volunteer Missouri Militia. I think it's just another name for "Missouri Militia." There were two types of such militias, both formed near the end of the Civil War.

The URL below will take you to an overview of different types of union militia organizations that existed in Civil War Missouri. It was complied by the St. Louis County Public Library.

My great-grandfather was in a "General Order No. 3" Missouri Militia Unit that went by the name Cochran's Bollinger County Volunteer Missouri Militia.

http://www.slpl.lib.mo.us/libsrc/mocivilwar.htm

George Williams