Are you talking about the belt 'bounce' and resulting vibration felt through the scooter?
Have seen the same thing on Burgman 400's - when the CVT/belt is slowing after a speed run....it bounces and flexes and causes vibrations.
If the belt is with-in specs (= correct width size) and not stretched from many miles - there might not be anything you can do about this with these 'rubber-band' powered scooters - except perhaps ease more lightly, more gradually, off of the throttle when slowing.
Otherwise I would suspect an issue with a belt worn past it's prime ....and fit a new belt.
Stig