wards are allowed anywhere, so long as they follow the following:
1) o not touch the objective (this is hosties when they're under T control, bombsite, or bomb once own...etc)
2) do not block off all access to objective. at least ONE path must be completely clear to get to the objective.
3) not in enemy spawn until 20 seconds have elapsed after freeze-time.
so, a ward in the vent is only not allowed if it's close enough to touch hosties, or if both other entrances to that little "house" thing are also blocked by wards.