EJ,
I can't tell if you are jesting about Vicksburg but to be sure you can rule out Vicksburg as that was in the western theater of operations whereas the 15th NCST were engaged mostly in the east, in Virginia. According to one regimental history it was posted in Richmond during the Pennsylvania Campaign so that might be eliminated as well. After you have this flag made please share it with us.
Cheers,
Mike