Whether the diaphram port is open or closed depends on vacuum in the CCV , not inlet manifold vacuum.
1 When the engine is started the CCV valve chamber is at atmospheric pressure, no vacuum.
2 The spring is pushing the diaphram away from the center port that is connected to the inlet manifold. So, there is an open passage from the crankcase, through the CCV to the inlet manifold.
3 The open passage allows manifold vacuum (engine now running) to suck air & blowby gases from the swirl chamber through the center port, eventually creating a vacuum inside the regulating valve chamber (and crankcase too, of course)
4 Now the diaphram will be pushed against the spring by atmospheric (higher) pressure on the other side of the diaphram. As the vacuum increases, the diaphram moves closer & closer to the port connected to the inlet manifold.
5 Eventually the diaphram compresses the spring enough that it touches the vacuum port and seals it off. This stops the inlet manifold from sucking any more from the swirl chamber. For an instant the vacuum on the swirl chamber is constant.
6 Next, blowby getting past the piston rings enters the swirl chamber & moves up into the regulating valve chamber which reduces the vacuum.
7 With reduced vacuum, the spring is able to move the diaphram and open the vacuum port to the inlet manifold again
8 Vaccum inside the chamber is again sucked down - around & around it goes.
So the vacuum in the CCV, and crankcase, is generated by inlet manifold vacuum and controlled by the CCV regulating valve. As long as the CCV is operating correctly, inlet manifold vacuum will always be greater than CCV/crankcase vacuum.
The size (diameter) of the diaphram, its flexibility/stiffness, strength of the spring and the relative position of diaphram & center port are worked out by the designer so that diaphram is just touching, i.e. closing off, the center port at a vacuum of 10 to 15 millibar (4 to 6 inches water column) in the CCV.
Manifold vacuum and crankcase vacuum, or pressure, are applied to the same side of the diaphram. Except of course, manifold vacuum is blocked off when the diaphram moves far enough against the spring to touch the port connected to the inlet manifold. The port from the swirl chamber (crankcase) is never blocked. The diaphram isn't molded in a special shape to block off both ports. The flat center of the diaphram moves to touch or not touch (block or open) only the port in the center of the the regulator chamber which is connected to the inlet manifold.
This is the crucial point; crankcase vacuum (or pressure) is always present. Inlet manifold vacuum is "switched" on & off by the diaphram to control crankcase vacuum at 10 - 15 millibar.
Perhaps it will help to point out that when the diaphram opens the manifold vacuum port, crankcase vacuum does not instantaneously go to to full manifold vacuum. Rather it begins reducing crankcase pressure (increasing vacuum) as blowby gases are sucked into the inlet manifold. As crankcase vacuum approaches 10 - 15 millibar, the diaphram blocks the manifold port again, limiting crankcase vacuum.
So, the vacuum at which the diaphram closes off the inlet manifold port (i.e. center) is crankcase vacuum. See this thread for measurement methods
Posts 7 & 8 suggest specific methods to measure this vacuum (or heaven forbid, pressure.)