What documentation do you possess which places a Vicksburg Cemetery tombstone for Pvt. Hoy?

I would assume you refer to Soldier's Rest. However, neither the UDC records or Webster's work list him.

I remember seeing about a dozen private stones near a tree just to the rear of the 77 stones we placed in
1998, but have to picture of them with names.

There is the possibility that Hoy rejoined the regiment at Vicksburg but that fact should have been registered on the subsequent Company F muster roll of February 28, 1863.

Here would be the possibility that he could have been recorded on the regiments Quartery Reports of Deceased soldiers for the 4th Qtr. of 62' or 1st Qtr. 63' Would require a visit to the NA however. Might be a remote possibility that duplicates are available at the Louisiana State Archives.