I did this swap last year on my '84 Vanagon GL and will probably do it again
this year on my '91 Carat Vanagon (I just got the replacement hose from Chris
Turner).
I have talked to a bunch of people out there and the general concensus is:
1. Change all o-rings,
2. flush lines and compressor of all oil,
3. use Ester oil and refill system,
4. change reciever/dryer,
5. Install new fittings (any Pep Boys or Napa has these).
5. Pull a hard vacuum on the system (let it sit for at least 30mins like this)
6. If the system is not leaking then recharge with 4 cans of R-134a (Napa
around here has a really good price on this stuff!)  You can charge a little
then charge the rest with the engine running and the compressor on.
7. Observe the guages to be sure that your pressure is not going too high or
too low(that means your expansion valve is bad).
8. Check your temperature at the evaporator with a thermostat.  It should be
a good difference from the outside temperature (30 degrees maybe a little
more).
If everything is good up to there then you are in business.
I am getting some parts in to make this easier and cheaper.
Like new dryer for 86-91 Vanagon for $60 (these are expensive)
'87 and up condensors for $120
and some other goodies that should make getting your Vanagon ready for the
summer less painful.
Have fun and it is really worth it.  If a hose blows or it leaks out from
sitting go down and get another $20 worth of R-134a and pump it back up and
you are set again (Instead of $120 worth of R-12 that will just leak out
again this summer).
Ken Wilford
Van-Again
John 3:16