Predicting what will be on fleet so far in the future is more or less impossible.

Hertz currently have the C6 Corvette Convertible on fleet, but these cars are already between 15 - 24 months old, so they will most likely be gone long before September 2015. What they will have by the time of your rental is anyone's guess. Although you might hope that they would have bought some C7 Convertibles, thee is no guarantee around this.

I suggest that you monitor the website as that will give a clue as to whether they have got new C7s or not and then nearer the time of your rental, contact them to find out what car they would likely allocate to you.