So, the first vibrato technique we will look into is what I call the "door knob" vibrato. The term refers to the rotation of the hand, rather than the fingers creating movement and exacting force.
In this case there are two main things to always keep in mind; 1.) That you anchor your left hand index finger knuckle against the neck (the high E-side), and you should never loose contact with the neck. And, 2.) that your vibrato finger wshould be almost completely motionless in regards to bending and extension of the joints. All movement should come from the turning of the wrist, rather than the fingers.
This will help you achieve a very controlled and even vibrato, since locking the fingers and rotating the hand will provide a stronger, and more controllable technique.
Check out both the regular, and the closeup vids if you are feeling uncertain as to how the vibrato should look and sound. AND, always practise any of these vibratos with ALL your fingers.