My cat Popsie has been neutered for 17 years and he still does the vibrating tail thing. I'm pretty sure it's part of a spraying behavior. Even though the cat doesn't physically spray it still goes through the motions.
i always heard that the quivery tail/butt display thing was a leftover kitten behavior; you're the surrogate momma, so he gets your attention with the tail, then presents his butt for you to 'clean' him
My orange tabby does this all the time too (no spraying), and his rear end is facing away from me. He's came to me from the shelter at 4 mos. old. I always wondered what it meant too. Thanks!