Scott,

I had exactly the same symptoms on my '90 Vanagon.  I too thought it 
was a vacuum leak (as yours probably has.)  Mine turned out to be a 
set of A*tolite spark plugs (platinum) that the tabs had nearly burned 
off, to nearly three times the gap at spec, after about 10K miles.  I 
replaced them with some Bosch platinums and the problem was fixed.  
Your problem is still likely a vacuum leak, but you still might want 
to check the plugs and rotor.

Dan Fuller