That's interesting that the vent came off. I've been driving school buses for years and there's no reason to worry about it flying off. Now, if it's not secured in the emergency handle part of it, yeah, it definately will come off.
My unenvironmentally-friendly testing (with no children on of course) shows that maximum airflow occurs when the back third of the bus has all the windows down, the front of the rear top vent is popped open, and the front door is open. When you're at highway speeds this creates effecient reverse airflow out the front door which helps meet the requirement of sweeping out the bus every day.
Also, if you're having problems defrosting the front windshield, open the back windows and the driver's window. This will create enough reverse airflow to keep the windshield clean.