I have had excellent luck using the Prestone flush kit.  I cut a tee into
one of the heater hoses underneath the Vanagon.  It makes an excellent
point for draining as well as flushing.  When you cut it in, have several
buckets handy.  Recycle according to local laws.  (I do believe down the
toilet is OK.)  After it is drained, I open the pressure cap in the back
and hook up the garen hose to the tee.  Turn on the hose and flush it out.
You can run the engine with it hooked up.  Once everything is clear, shut
off the engine, remove the hose and drain from the tee.  Close up the tee
with the cap that comes with the kit. I then start filling with straight
antifreeze in the back.  Two gallons I think is about right.  Once I get
that in, I continue to fill with water.  Open the radiator bleed until
coolant comes out.  Close it up.  Let the coolant get warm with the
pressure cap off, then screw on the pressure cap with the engine running.
(Otherwise it will back up and overfill the tank.)  Continue to run until
hot keeping an eye on the tank level.   Check it for the next couple of
days and you should be good to go.   I'm sure I forgot something, but
someone will certainly remind me if I did!
I think the reason the WBX's fail is this critical element.  Most people
just don't change their coolant as often as they should.  Coolant is
relatively cheap.

                                                        Jay