It is NOT OPERATED BY VACUUM!
It is operated by AIR PRESSURE.
Manifold pressure is fed to that capsule all the time.
At the pre-set upper limit of boost, the pressure in the manifold is enough to overcome the spring pressure, and so the diaphragm in the capsule shoves the rod and that swings the lever which opens the waste gate valve.
This slows the turbo down a bit, so if you keep the foot down it "balances" with the turbo producing maximum pre-set boost, usually about 1 BAR / 15 psi, enough pressure to blow up tyres.
OK?
CharlesY