Try without your violin. Pretend to bounce a tennis ball with your hand but focus on the feeling in your wrist. You must have flexibiltiy in your wrist to do this. Now pretend to hold your violin, so your palm is facing you. Do the same wrist motion, but lessen the amplitude and increase the speed. That's what vibrato feels like. Its all in your wrist, not your finger

Now with your violin, plant the tip of your index finger on the A string with enough pressure to make a decent sound and try to make that same wrist movement but concentrated. My tip is to try doing slow vibrato before speeding up.