I have done this several times on Vanagons.  Usually a good shop with ALWAYS
replace all the of the o-rings when replacing a compressor (just common
sense).  You can get all of them from NAPA for $10 or less.  Just take your
old ones in and match them up.
Also inspect your hoses for wetness as these can go bad and seep through.
If you still don't fix the problem by replacing the seals and all of the
lines are good then take it to a shop that has a leak detector or use some of
that leak die to trace it down.
Too cold to think about that here in South Jersey, just yet.

Hope this helps,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242