These are prone to get corroded internally.  If you want a cheap fix, try
removing them and cleaning up the inside of the unit (not all that easy but
doable).

The other thing to try is to be sure that your contacts on the sliding door
are clean and aligned properly.  Mine were out of alignment and this caused
my sliding door lock to not lock sometimes, or not unlock other times.  The
latter would cause the rubber-band effect that you discribe.  What I would do
to fix it is lift up on the door handle while operating the door lock in
unlock mode (this would allow the contacts to line up) and the lock would
unlock.
If you can't fix it either way I have new front door lock units for $70 each
and sliding door and rear hatch units for $65 each.

PS Bulley the Vanagon power locks don't use vacuum at all.  Only electricity.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242