to fix it, drop the engine, get the correct torque convertor bearing from
VW and the correct seal.  install the seal properly or it may just get cut
up in the process necessitating doing the whole thing over again.

it wasnt too hard on my 80, i would guess that even though your 91 has a
different AT it would be a very similar procedure.

if i am wrong, please flame away :)

In a Vanagon, it is much easier to remove the Transmission to do repairs on 
the torque converter or clutch.  Look carefully for other sources of leaks.  
This seal rarely goes bad.

Dennis H