You're talking about a spoiler? I never saw one on a vanagon, maybe something like what some suburbans have would work. It's a near vertical piece with a slight curve to it and it scoops air from above the car and shoots it down along the back of the vehicle.
I put a spoiler on my rabbit, the kind that would give the air coming over the top a slight kick upward. The idea was to keep the back window clean, didn't work for that but it looks good so I left it on, the 3M two sided tape has held good for over a year now.