It might not be possible to hook up any old tachometer.  The engine uses
a Hall-effect distributor, and finding a place where the current looks
like an old distributor may be difficult.  I use a tachometer that has
an inductive pickup you clamp around one spark-plug wire, like most 
timing lights use as well.  These usually work, although there is a recent
trend towards multiple sparks per firing, which may screw up even this 
variation on some newer cars, or cars retrofitted with an MSD ignition (not
to worry, I doubt anybody'll make soomething like this for Vanagons!).  

The stock tachometer on your van hooks up to a wire on the coil -- Number 1
I think.  Try this spot -- VW seemed to have good luck there.  But many cheap
or old tachometers for engine-tuning don't work so good.  A small-child-
based audio-relay system from the stock tach is usually better.  Have any
components around to scrape together one of these?  Neighbors often will
lend this particular tool, even when they won't lend a screwdriver.

malcolm