Originally Posted by trickg

That's not vibrato. It's a lip trill or a shake. In Maynard's case, he's moving smoothly and rapidly between two partials on the horn, usually up to the next partial from the note with the shake or trill.

Some people use the tongue to do this, some people actually shake the horn. I use a combination of both. (probably NOT the best practice, but it's what I do and it works ok)

You aren't the first person to think that it is just a wide vibrato - I thought the same thing after my first few listens to Maynard and Phil Driscoll until I tried to reproduce it and found that it was much more than just vibrato.