I don't really follow why resetting the trip odo while at speed should be a
problem; when you look at the odo-guts, you'll see that the mechanism is set
up to pull the trip gearing out of mesh and THEN zero it.  Maybe the shock
of bringing it back up to speed when it meshes again.  Anyway, what breaks it
either the grip of a little gear on its shaft, or the gear itself.  If the
gear breaks, it turns from an O with teeth to a C with teeth, and the teeth
next to the opening aren't spaced right any more and they jam and besides that
the C doesn't grip the shaft as well as the O did.  Comes the time the gear
is asked to pull a couple of 9s in addition to the regular tenths, it sits out
a few, and then you wonder why you only got 49 miles on that tankful.

Wisht I lived in Vanagon-rich CA.  Wrecking yard Vgons are very sparse in this
part of the world.
B.