Years ago I recall reading somewhere that the hospital ledgers from Chimborazo Hospital in Richmond (world's largest, ever) still existed. If Vandiver was treated there, perhaps his fate was recorded. Don't know if the National Archives or the Museum of the Confederacy has this collection. I'd still check war-time court records in Jackson County, GA for any reference to his death. The state archives in Morrow, GA have this documentation on microfilm.