Musing over conversions, one of my many talents, brings me to consider the
cost-benefit scenarios.  Let's postulate that we drive 10000 miles per year
just to use easy numbers.  Discounting the normal maintenance for both a
converted vanagon and a non-converted vanagon, the costs of conversion seem
to range from ~$14000 (not a misprint) in my case to perhaps $3000 for an
average of $5000 (high but we have to start somewhere).

If fuel costs over time are the principal reason for going to diesel (not
fuel availability, or the desire to side with Willie on the BioDiesel
concert) then it makes sense to look at the fuel savings one would enjoy
over that 10000 miles per year.  Diesel currently costs about %10 more than
87 Octane and is likely to stay that way.  For $2.50/gallon 87 octane let's
say and vanagon average fuel efficiency of 17 miles per gallon.  The gallons
per year is easily found to be ~588 gallons times $2.50 gives $1470 per year
fuel cost before conversion.   If you go with diesel the fuel consumption
will probably be on the order of 25 miles per gallon.  For the same 10000
miles per year you'll pay $1100 in fuel costs including the %10 higher price
for diesel.  You will get additional range between fuel stops but you'll
need it as not all fuel services feature diesel.

In summary you'll save $370 per year in fuel costs for 10000 miles per year.
How many years to break even on the installation is left as an excercise for
the reader.

Seems to be over ten years if your previously owned motor lasts that long.
FTSOE let's say it does.  You will have saved $3700 towards the motor
replacement.

The above example is to be considered a conservative estimate process, your
mileage, tolerance, costs, will no doubt be different.

"If a man professes knowledge but cannot express that knowledge in numbers,
then that knowledge is of a meagre and insufficient kind"  -- Lord Kelvin

Numbers rule!