Normal Bondo shrinks over time (hence the cracking), there is another filler
that is mostly aluminum and doesn't shrink as much (I think it is called
All-Metal or something like that) I am sure your local body shop would be
familiar with that.  However the best way to go is to replace the door
completely with a good used one and have it painted to match.  This is the
only permanent repair that I know of.
I just found out that the door of the Vanagon was changed in '85 some time.
To be safe only get a door from '86 or newer.
Hope this helps you.
Ken Wilford
Van-Again
John 3:16