Recently I've been trying to work on my bends and vibrato but I can't really seem to the best way to do it. I usually play with my thumb behind the neck, so the first things I tried were to bend using only my fingers and to vibrato by either shaking my arm downwards - upwards doesn't seem to sound too good yet - or by bending fast. But the bends don't sound consistent enough and the vibrato lacks the absolute control I want, i.e., it only has one tempo.

So now I've been trying to do bends and vibrato by putting my thumb over the top of the neck and using a "turning a doorknob" motion with my wrist and both seem to be much more consistent and controlled...but my thumb has to move. So if I wanted to play something like Jason Becker's Altitudes, currently I would have to move my thumb over the neck every time there is a bend or vibrato, then move it back for the fast alternate picked parts, etc. Will using two different left hand positions make songs like that impossible to play? Or will the switch become much faster over time? Or is there another way I should be doing bends and vibrato?

Thanks UG
When I do vibrato and bends, I usually grip the neck with my thumb and "turn the doorknob".

And yes, the switch will become faster over time.