Based on some things which I have learned here in Fall 2006, and some new things I discovered while teaching, this post will elaborate on vibrato.
A. Vibrato Speeds
Those who have problems slowing down or speeding up their vibrato can try these steps - especially in slowing down vibrato.
1. Make sure that the wrist is free - sometimes the problem comes from doing arm vibrato when wrist vibrato is intended.
2. Try keeping the other fingers free - if the other fingers are loose enough, the wrist will likely be more free as well. In the end a lot comes down to having a good violin/viola holding position - something we all need to work on, myself very much included.
3. Try taking apart the vibrato - play just one vibration like this:
Check that the vibrato is a smooth, consistent motion, and try it very slowly, then faster. Use a metronome and time how long the pitch movement takes. Then try two vibrations, three, etc, at faster tempos.
B. Other Areas
3. Be able to start vibrato from the moment that the finger hits the string - good to have this as a default position.
4. Match the vibrato with the bow speed - if the sound isn't what is in one's head, maybe it's not only a vibrato issue. Try matching the bow speed with the vibrato.
With thanks to Cheng Hou and Tony Devroye, currently at Northern Illinois University.