Here is a very interesting collection of articles about vibrato.
I've found that one of the hardest things to do to a student is to slow down a fast vibrato. What you basically have to do is ignore the learned vibrato and start again from the beginning. Sometimes it may even seem easier to start a new vibrato from the right hand! Of course that involves a host of problems, restringing your instrument, resetting the bass bar, learning to play left handed....
