Since a couple of weeks i take special exercises for vibrato. Allways played them but never took real attention to it. It's difficult to explain how do do it, play it from your wrist not only with the fingers. Now i exercise vibrato with a metronome. Making half notes, quarter notes, triplets etc. in the rythm of the metronome With all four fingers on every string. Before starting playing/exercise i do this for about 10-15 minutes every day and slowly increase speed. Then play along with backing tracks and other artists and listen and play vibrato's. Listen carefully to your own vibrato.