I have not seen anything to suggest that Alonso or Hamilton would not have been able to match Vettels achievements.
That said, it is an interesting conundrum, especially as there is little more than tenuous links to compare in a like for like manner, hopefully that will be remedied in the years to come, but I think that the Vettel, Alonso and Hamilton are generally considered to be the top three for a reason, and I see no reason why things would have taken a different turn whichever of those three were behind the wheel.