micra said
read something good today from an article by Simon Fischer on the internet, it has made a difference for me, practicing so long with the metronome, and unable to speed up the vibrato:

"Vibrato mechanism
Of all the many issues concerning vibrato, the most common difficulties arise from the following:

There should be only one active direction of movement in vibrato: forward to the in-tune note.

1. Vibrato can be likened to clapping, where there is only one active movement to bring the hands together, the hands then bouncing apart again as a "passive‟ movement. The hands go "in, in, in, in‟ not "in, out, in, out‟. In the same way, vibrato goes "forward, forward, forward, forward‟, not "forward, back, forward, back‟.
   Trying to make an active movement forward and back requires twice as many actions and thus requires twice as much energy. It creates a vibrato that is too wide and too slow, but efforts to speed it up may end in fatigue and tension.

1. Finger pressure should be heavier during the forward movement, lighter during the backward movement.
   During the forward movement of the vibrato, the fingertip naturally leans slightly more heavily into the string. During the passive, backward movement, the finger naturally releases the string slightly. A vibrato with equal finger pressure backwards and forwards is impossible to play fast, and is a major cause of left hand tension in general.

There are specific exercises designed to build the rhythms of the vibrato, or to release the finger on the backward vibrato movement, which can be very helpful."

 

That's exactly right.  

VIOLIN VIBRATO: KNOCK AND THE DOOR WILL BE OPENED

1. Make a loose fist with your left hand.  

2. With a straight wrist hold your knuckles (those nearest the tips of your fingers) about 1 or 2 inches above a hard surface like a table or desk top.

3. Keeping your forearm still, rap your knuckles repeatedly on the surface bending the wrist to do so. 

Notice that all of the force is downward toward the table and that the wrist automatically rebounds back to straight after each tap.   That's essentially the same thing as a vibrato movement.  It is not a conscious push the hand forward, pull the hand back motion at all. It's a strike and rebound motion.

Move your hand a little higher above the surface and do the same thing, then a little lower.  Then do it more slowly, then more quickly. Whether you tap your knuckles from a higher point, which is equivalent to wide vibrato or lower point, which equivalent to narrow vibrato, it's the same thing.  The principle is also the same whether you do it fast or slow.

Next, keeping your hand exactly like it was, turn your hand around until the palm is facing you at the distance it would be on a violin neck.  Now repeat the motion that you just did on the table -- that's the motion for wrist vibrato.  

Next, with your palm still facing you repeat the knocking motion, but this time keep the wrist more or less straight and instead knock from the elbow.  That's arm vibrato. 

I'm pretty firmly convinced that most people that try to teach vibrato don't actually have any idea how they do what they do -- and after you can do it, you might not either.  It's like riding a bicycle, once you can do it, it's hard to understand why others can't and trying to explain it to someone else doesn't often work.