You are a lucky person that the AC vents have gone stiff otherwise usually they become freely moving and doesnot stay where you set :).
By the way go to the service station and ask the person to blow some pressurised air onto the AC vents in such a way that the air should pass through it from every directions. Also turn the vents in all possible directions during the above process. All the dust particles will vanish and your AC vents will be freely moving.
One more thing, the left-right movements are on the central strip of the vent, try to clean the central strip with a wet cloth so that there should be no friction of dust during left-to-right or right-to-left movement.