They probably did fix that leak.  What happens is when your coolant system is
leaking slowly it takes pressure off of the other lines and components.  When
the leak is fixed then this pressure comes up and then next weak component or
line dies.  That is why it is a good idea to replace all of your coolant hoses
at one time (since if one goes bad then the rest can't be any better).  I know
this is usually not practical on a Vanagon with the high price of water hoses
though.
Drop the spare tire down and take a look up there to see where the leak is
actually coming from (there are lines for the heater core up there and also
the coolant hoses for the radiator).  If you still can't see it then get
someone to sit in the van with thier foot on the brake and start the engine.
You should be able to see where the leak is coming from then (especially if
they rev it a little :)
Have fun and don't get wet!
Ken Wilford
Van-Again
John 3:16