Vents closing by themselves
I too had the same problem. When installing aftermarket speakers in the doors I fixed it. There is a small spring-metal tongue about 1/2" wide with a detente tip that will "click" into a notch on the bottom side of the vent when you pull it towards the interior of the car. With time, evidently, they bend back enough to not keep the vent in place in the up position.
One of those things that drives you nuts.
I don't know if you can do it without taking the door panels off, but what you need to do is GENTLY bend the metal tongues back towards the vents, from the inside. I did it when I had the panels off, so it wasn't too bad. I'm inclined to think it would be tough to do with them on and unfortunately the vents don't pop out altogether so there's no easy way to get at the tongue from the top end.
Good luck! In the meantime, wedge something in there...
Don in Germany