Its common problem, i find pushing the trailing edge of the door in just as it closes removes the need to slam it to hard.
The air pressure problem is made worse if you have lined the rear of your van, behind the rear panels/wheel arches there is an air vent in the floor that allows a bit of air through to help out slightly but if you have replaced the rear panels with solid ones it seals it up a bit (thats why the original panels stop short of the floor/have holes in them down low)