The classic things you learn from anywhere is that vibrato comes from healthy singing technique and good breath control. Singing gently, you can attain a nice vibrato without having to move your mouth at all.
But when I watch theatre performers sing, they 'quiver' their mouths almost as if it helps produce the vibrato. But these are professional Broadway-level performers so they are obviously not producing it artificially. I've not heard of this technique before. Is this something exclusive to theatre?