SuperTed,
It has got nothng to do with static pressure or the capsule.
A vibrator in an altimeter is there to overcome stiction / friction within the mechanism, i.e. cogs and a suitable system of levers and pulleys etc between the capsule and the indicating needles.
Without it an altimeter needle may lag / jump and operate in "steps". A vibrator just smooths out the movement of the needle. Note that it isn't often necessary to fit one to helicopters or piston engine aircraft that vibrate through other means.