"I suppose he could be among the unknown buried at Vicksburg but as I understand it most of those would have been battlefield casualties."

Not so, he could very well be buried in Cedar Hill, Confederate Rest Cemetery, although not recorded on extant records. See: http://www.nps.gov/vick/historyculture/soldiersresthis.htm

There are still over 1000 unknowns buried there.

George Martin