Ken,
I've turbocharged various vehicles and made turbo manifolds (that's the
hardest part) see
this page for a Honda manifold I make:
http://www.telusplanet.net/public/gary2a/honda/hondamanifoldahort.htm

With the vanagon here are some issues that come to mind:
- IMHO, WBX will not stand up the added stress and heat, and I don't think
it can be beefed up enough either.
- Digifant, Digijet, CIS, none of them will provide enough fuel under boost.
You'll go lean and melt pistons.
- Even with the low compression 9 to 1 WBX you will have problems with
detonation.
- The 10:1 RD or HT inline 4 will be impossible to keep from detonating.  A
knock sensor is not a solution, lower compression is.
-  The oil return on the vanagon is a problem.  I've seen a guy use a drain
tank with a power steering pump to evacuate it, a kludge at best.

It's been done before on the vanagon, but with all turbo conversions its a
never ending project.

Gary Lee
http://www.telusplanet.net/public/gary2a/rack/vanagonrack.htm