Yes, you could cut the hole and install the vent and later drop in the antenna and hook the wires. If you or your carrier would feel better, some silicone/latex caulk could help maintain the position of the antenna in the hole. I'v seen a couple of slightly different shapes/sizes, but only suspect the holes are same diameter.
Cutting the hole is as simple as can be. I used a metal blade in a basic jig saw. Anyone with a jig saw can do it so save your jobber money.
If for some reason your carrier doesn't like the idea of an antenna in a hole in a vent in a roof, you would still have a vent for the rear of the van and that's a good thing. whatever you do, place the antenna as far back as practical so you won't hear the antenna searching for the signal as you try to sleep.