Had this problem on mine several times.

How many miles do you have on it?  How many times a day do you crank it?
In the past, working delivery jobs, I often started the engine 150 to 250 
times a week.  Stop and go, stop and go.  

The starter has been rebuilt at least 3 times (the poor engine has over 
250,000 miles on it WITH NO HEAD LEAKS!!!) and I've changed the starter 
bushing in the transmission twice.  If you have chronic slow starting I 
would do both of these things.  

The van, now my fathers, still cranks slowly sometimes, but overall works 
ok.  I've seen this problem on many vans, I think VW may have 
underestimated the torque requirements for the vanagon starter motor.

Good luck,
Trace Choulat
tchoul@unf.edu
tchoula@junix.ju.edu
-- '93 MV