I will quote from the Parts Place catalog.  "VW HEAD REPAIR IS TRICKY"
"Many things can go wrong.  The head can be warped.  The center of the
head is higher than the ends.  When you bolt it down there is a space or
gap in the center and the head leaks between the two center cylinders or
between the the cylinder and a bolt or oil or water port.  The typical
machine shop "grinds" the head or surfaces the head.  This means if the
space or gap is 0.010" then they take 0.010" off each end until each end
is level with the middle again.  Sounds simple BUT the entire head is
warped, not just the head surface so when you bolt it down again you are
bending the cam up 0.010".  If you are leveling the bottom, the top cam
will be bent.  This is just a little thing which the typical "machine
shop" forgets.  

Now, my thoughts.  This is Parts Place and they may just be trying to sell
you new or rebuilt heads, however I doubt that.  This article also
appeared in the section that covered in line four cylinders and may or may
not be applicable to the opposed fours such as the Vanagon.  I am not an
expert in head repair but, it tends to reason that the heads are probably
made from the same material in both inlines and opposed so that a warp is
a warp is a warp.  Please do some research before you let the machine shop
touch your heads. Good luck.