Here's a great vibrato exercise...assuming that you already have a fairly developed vibrato, this is a great way to gain even more control. With the metronome set at quarter note = 60, play 2 beats of straight tone, then 2 beats of tone with vibrato, then back to two beats of straight tone. You can practice changing the speed of the vibrato pulse, first with 4 pulses per click, then 5, then 6. Your tone should retain its lovely center and focus with and without vibrato, and the dynamic should match. This exercise helps create a heightened awareness of vibrato.. Enjoy.