well realistically the only hard part is the outer shell, but most of the integrity of the helmets is made up of the polystyrene (or what ever material they use) inner core. It's really easy to puncture the outer shell by itself. I don't think the vents create any less structural strength than full face helmets. F1 crew engineers back me up here? Kero? Shi?
structural integrity is comprised of the outer shell. the reason why there is the polystyrene or foam is there is for the helmet to fit your head and be able to deform a bit before it (the hemlet) enters your noggin in the event of a crash.
basically it goes in general: shell (absorbs/deflects most of the impact load) -> liner (absorbs remaining impact load) -> padding (for comfort and fit) -> your head.
the vents are at the front, if you're going to be smashed in the face its more likely that the visor will give before any other part of the helmet in the front.. now of course if you're taking a javelin to the face then you're SOL in the first place. that being said, it wouldn't be very difficult to add a few more layers of CF to reinforce the structure near the opening to make it just as strong as if it was thinner and without the big vents since most of the space is occupied by the foam anyways.