Hey, I just spent the winter in the rockies with my vanagon.  You def.
have altitude sickness.  it is running rich etc.  my suggestion is
retune it completely at high altitude (idle a touch higher than usual)
and then when she still won't crank unplug the fuel pump (you might
get good at it) until she fires then plug the pump back in and she
will crank right up...  foot to the floor on first crank doesn't hurt
either  (just don't redline it without good oil circ.)

good luck..

Jonathan Poole