While maintaining a steady, relaxed, warm, rhythmically controlled vibrato on any finger anywhere on the fingerboard, do all sorts of unrelated movements with the right hand:
1: without holding the bow (tap your head, scratch your leg etc)
2: holding the bow, but without it touching the strings
3. bowing on the strings
- Â with the bow always in contact with the string (portato, legato bow changes, staccato bow changes, short accents, fast whole bow strokes etc).
- Â placing the bow from the air, and bouncing bow-strokes (spiccato)
For example:
Practice starting the vibrato before the bow-stroke starts and also maintaining the vibrato after the bow-stroke has finished. Do these exercises on all different fingers, on all 4 strings, all over the fingerboard and with a metronome (to control the vibrato speed).