The vacuum is part of how a cyclone works, another way to visualize it is that the fan creates an empty space by pushing air out the exhaust, the air that is outside the system tries to rush in to fill the empty space that is inside the system.
If there is high resistance to flow (small dia pipe, long length of pipe, lots of fittings, long flex hose, etc) the air can't get in as fast as it wants to so this gives us a high vacuum number, with the system inlet fully blocked (the spade) this gives us max vac but no air flow, the opposite is fully open which gives us max flow and a low vac number, to take Bob's value of 10" and lets say that was measured at the fan inlet but we then took another reading at the cyclone inlet it might read 8.5", What happened to the 2.5"? It is "consumed" by the internal resistance of the cyclone.
Each metre of pipe/hose, type of fitting, all have a value of loss (back pressure) the more stuff we hook up increases the back pressure and if you hooked enough stuff or did something like put on a bit of 32mm dia hose, the original 10" is all consumed and you get very little flow, (the spade), so if there is minimum flow there is minimum air speed inside the cyclone and little centrifugal force so particles are not held out against the cyclone wall and remain entrained in the airstream and out the exhaust they go, if the air speed is high enough the particles are separated from the air stream (centrifugal force) and held out on the cyclone wall, friction and gravity do their thing and into the bin they go.
Pete