Mike,

Easy to change status to diesel with the DMV.

Conversion to diesel will cost you $4-$10k + a lot of hours of your time.
How many miles will you have to drive to recoup $4k-$10k??  Do you
really want the hassle?

It might be smarter to buy an older fuel efficient car (Corolla,
Civic, Geo Metro etc) and use that vehicle wherever possible.  Save
your Vanagon for special occasions or when you truly need the room.
This approach will also save wear and tear on your Vanagon.

I have many Vanagons but my main daily driver is now a 91 Corolla.
Well under $2000 for a really good one, runs like a champ, has great
AC, and consistently gets 30MPG.  Corollo's/Civics from the early
90's are also very easy to work on and parts are cheap.  I know it's
not German but it's reliable.

just my $4.59/gallon I mean my 2 cents