Does it vibrate only when a belt is on it? or does it also vibrate when the belt is removed and the motor is running free?
If vibrating when motor is running free check motor bearings.
If vibrating only when belt is in place check idler wheel bearings.
Look for buildups on all rollers and remove if present.
My bet would be bad bearings on a small diameter idler wheel, these spin awfully fast and wear out quickly. If you spin an idler wheel and bearings make any noise at all replace them.