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"

When you eat dirt for the first time and leave your family to build a shack in the woods