I have a 2nd Gen but took out the vents, covered them with sheet metal and caulk. Then sound deadened over everything. I don't see any reason to have cab vents if you are going to put in subs. I want to build pressure in the cab with my subs. SPL is fun.
The only thing I've ever noticed is the doors are a bit harder to close than it was before the vents were gone. Outside of that, every car/truck I've owned was fully sealed/sound deadened and I've never had issues.