Hopefully I'm not dredging up a thread that's too old here, but...
When I was a freshman in undergrad and still trying to do a viola major in addition to composition, I had a teacher who strongly believed that one needed perfect technique before one can make music. I rather disagreed with this perspective (I believe that one can have great musicality even if one doesn't have the world's greatest technique), but stuck with the teacher for a year while he tried to completely revamp everything I'd learned thus far. One of his particular points of focus was vibrato, and he gave me a whole arsenal of exercises to do. I found them annoying at the time, but now that I'm several years removed from said teacher, I've realized that the exercises really were helpful.
I can try to describe them here, and if you'd like, I could pull out the digital camera and take some pictures of the motions.
This teacher focused on the fingers as the main source of motion in vibrato, and he mentioned that the wrist and arm follow what the fingers start, when you need to extend the width of the motion past the fingers. Therefore, these are mostly finger-based exercises.
Hold up your left hand. Keep your index finger up, but flex the other fingers back and forth ten times. Repeat this with each subsequent finger held up and the other four flexing. Then do the same motions but with each successive finger kept down while the others flex. The finger-down version can then be transferred to the instrument - one finger holds its position on the string while the others flex up and down.
Hold up your left hand, and rest the tips of your index and middle finger on the tip of your thumb, so that the two fingers are bent at the same curvature you'd use on the instrument. Keeping the middle finger still and making sure that both fingertips remain in contact with the thumb, wiggle the index finger side to side ten times. Then do the same with the middle finger. After that, switch so that your middle and ring fingers are the ones touching your thumb and repeat the same process. You can try with your ring finger and pinky, but you might find that your pinky is too short, like mine is. My teacher didn't seem too worried about the pinky in this regard. This is another exercise that you can then translate the motion to the instrument.
Holding the instrument, place your hand loosely in first position, and using your index finger as a pivot, rock the rest of the hand side to side. Repeat with each successive finger.
The nice thing about most of these is that they don't require being anywhere near a viola, and thus can be practiced pretty much anywhere. I've also found that they're good for stretching and relaxing my hands before I play.
I hope that's at least marginally helpful!