Looks like the link that Mike posted has this moisture barrier:
http://www.perfectfit.com/15347/154532/Chipboard-Car-Panelboard/Kraft-Tan-Wateproof-Panelboard.html

I can say that the vinyl covered composite material I mentioned also provides a moisture barrier.  I have used this in my business to mount canvas prints, and the material is absolutely stable, unlike masonite or hard board.  Any type of acrylic panel is also going to be completely waterproof.  I know that the front doors have a vapor barrier, and I certainly was not advocating not using this in addition to the door panel, but I'm not sure other panels in the van have this moisture barrier.  My sliding door panel did not.

Troy
  ----- Original Message -----
  From: Stacy Schneider
  To: colorworks@gci.net
  Cc: vanagon@gerry.vanagon.com
  Sent: Tuesday, December 28, 2010 10:00 AM
  Subject: Re: Replacement door panels

         The upholsterers actually have a treated  cardboard they use . it has a waxy side that is designed to go up against the door . Even with that said ,you still need to put a vinyl vapor barrier on the door before you seal it up . It keeps the moisture of of the door panels and prevents drafts . Contrary to popular belief the scrapers on the door top do not keep water out . They are designed to stop large objects which plug the door drains.

                                                                                                                                                                    Stacy