Choir slang for a slight, usually uncontrollable and involuntary bobble of the head and/or jaw while singing. Considered cool for some styles such as pop and jazz but irritating in genres such as musical theater and especially in film musicals. Vibobble can be considered cute if involuntary but is usually a bad thing if exaggerated or done on purpose.
From the words "vibrato" and "to bobble"
-That guy playing Marius had some serious vibobble going on.
-Aww man, if I can't learn to control my vibobble, I'll never make it on Broadway.
-I love the way his head vibobbles when he hits that high note.