Sky - vibrating the contact is a big NONO, it is also known as sawing at the bit (no matter how big the saw/vibration is) and doesnt get the horse to relax thier jaw, what it does do is teach the horse to drop behind the contact which FEELS like the horses jaw has relaxed. Result is over bent horse who's back is not up under the saddle and a very latteraly unsteady head carriage which undermines you when you try to teach latteral work.
In fact he has learned to accept contact and use his back. It's still not perfect but we've come far from where we once were. Which was a bracing evading giraffe horse mess.