Bob,
The bezel has a prong on one end that goes into the hole first, and rides
on the back side of the door panel, thus pinching the panel between the
prong and the face frame of the bezel.  The opposite end of the bezel has a
beveled tab that deflects as that end is inserted into the hole (similar to
a latch on a door), the tab then springs back once through the hole,
securing the bezel.  In my experience, the bezel is usually found to have
the prong on the top, the tab on the bottom, but it also fits if rotated
180 degrees.

It is often possible to remove the bezel, along with the switch, by pushing
up from underneath with a flat-bladed screwdriver (if the prong is on top),
while pulling out on the bottom of the face frame.  If the prong is broken
off, or the tab is "lazy", a new bezel will be needed.  See here:
http://www.gowesty.com/ec_view_category.php?id=203  Scroll down about 2/3
of the way on this page, and select either the single bezel, or the double
bezel, as needed.  Note: it is also possible that the door panel has been
damaged by improper removal of the bezel in the past, such that a new bezel
will not properly snap into place.

To actually change out the bezel, the approximately 6"-long wire leads of
the switch must be disconnected at the push connector, which is hidden
behind the door panel; you may get lucky in being able to pull the wires
out through the hole, to disconnect the push connector, but more often than
not it is necessary to remove the door panel. If you need instructions for
the removal of the door panel, either post again or p-mail me.  It's not at
all difficult, but if you haven't done it, a little instruction will make
it easier.

One last note: If you're dealing with the driver's-side bezel, remember
that the driver's side switch occupies the aft position in the double
bezel; I have seen more than one Vanagon with these switches reversed as to
position, indicating an error on re-assembly.

Bill