Starting with banjo position frees up the upper arm. Whether you use predominantly an arm (forearm) or a hand vibrato, freedom in the upper arm is important for a consistent vibrato.
Slide the finger along the top of the string. As you narrow the action keep the same level of energy in the movement but let the finger settle onto a note. Notice the balancing action in the thumb and the flexible nailjoint in the vibrato finger.
Paul Rolland and Sheila Nelson have shown the need for passive, balancing movements in string-playing especially in fast cyclical actions like vibrato. For instance in vibrato the upper arm shows a passive rotary movement which balances the action of the hand or forearm.
SEE
Vibrato - Introduction
Bibliography
- for The Teaching of Action in String Playing by Rolland, Ch.2
...