Jason,
This problem sound like a very similar problem I have had, and I
replaced the ignition switch, neutral safety switch, added grounds,
sheilded the starter from wheel wash,and replaced the starter 4 times. (
and replacing the switch was the biggest pain of them all.)
It seems that the Vanagon suffers from a voltage drop--sometimes--- to
the starter, and occasionally doesn't want to crank.
The solution, I was told was to add a starter relay in lne to the
starter.
However, I found that if I jumped off the positive side of the battery,
to the wire to the starter, at the neutral saftey swich, in the consol,
the thing woild always crank.
So, I added a starter button to the dash, so when his cantakerous
vehicle, decides not to spin the starter, I just push my genuine
Caterpillar starter button an BANG__it cranks, every time.
I have scatched my head on this one, for a long time, and have picked
the brains of many of my, guru, gear head buddies, and they always draw
a blank, as I did pretty much cover everything.
Befor you try replacing anything, try this;
Crawl under your van with a screwdriver in hand, and jump the two
terminals,          ( battery, and signal wire) and see if the starter
spins. If it does, I would venture to say, that you have a voltage drop
problem.

Good Luck,

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Terry 85 GL