I do understand that the cable sheathing does not support combustion. I am trying to determine at what temperature it will melt by fire impingement. Thank You.

If there is direct impingement, the melting point is irrelevant. A campfire is typically 1500*F.
Vicat temp for PVC, the softening point, and realize there are many different blends, runs about 190*F. We used PVC and CPVC coatings in a number of different circumstances. PVC (vinyl) siding is notorious for melting at fairly low temps.