I noticed that the Pooley's book suggests that a blocked inlet filter would result in reduced vacuum reading.
I'm not sure what a Pooley's book is, or it's authority, so can't comment about that part. Thinking your way through the circuit: The vacuum pump either runs, and produces a vacuum, or it does not - like a household vacuum cleaner. (It is possible for a vacuum pump to be really worn out, and produce less vacuum, but usually they fail before the vacuum drops noticeably.) The vacuum is pulled past a vacuum relief valve, (which has it's own foam filter). That valve is adjusted to prevent too high a vacuum being produced to the instruments. From there, the vacuum source is connected to the attitude indicator, DG, and suction gauge. So the vacuum pump spends it's whole operating life trying to take out air from these three instruments.
To operate, the AI and DG require air to enter, to blow as a jet across the gyroscopes (the vacuum gauge is just a diaphragm, like a very simple altimeter). To the AI and DG, that air comes from within the cabin. It is wise to not draw crap into these instruments, so that air is filtered. The instruments will, however, operate just fine with no filter, other than the risk of becoming gummed up. (AI and DG's have had much longer lives operating in the panel since smoking was disallowed in cockpits, they used to look brown and gummy inside from smoking planes). Considering the filter as not being restricting of the airflow, there is no other restriction between cabin air and the gyro jets inside the AI and DG.
If no air is allowed into the AI and DG (blocked filter), it'll be like holding your hand over your household vacuum cleaner nozzle, the vacuum cleaner will still produce a vacuum, just no flow. The pressure of the air in the hose is reduced to the efficiency of the vacuum pump. Note that "volume" type vacuum pumps (which the plane has) do not produce a high vacuum/low pressure, but rather lots of lower pressure air. The type of vacuum pump used for calibrating altimeters produce a high vacuum, very low pressure, to simulate high altitudes, and could wrap a vacuum system vacuum gauge right around, but would not produce enough flow to run even one vacuum gyro instrument.
So if the vacuum system air filter were to be blocked, no air could enter the AI and DG, and they could not operate for lack of differential pressure across the jets for the gyros. The vacuum pump will still draw air, so it will now come through the vacuum relief valve as needed to maintain the vacuum setting, and the suction gauge will still indicate that. So, the vacuum gauge is not an indicator of flow, or a filter obstruction. The suction gauge will indicate three things: A complete failure of the vacuum pump, as no vacuum at all, a failing pump, as a lower vacuum (Maintenance may be able to reset the vacuum relief valve to capture that last efficiency before it fails entirely), or, a case or hose leak in the instruments or plumbing, which allows air entry on the vacuum side, where it should not be. This could be a reduction in vacuum, or total loss, depending upon the size of the leak.
Most modern flight manuals show a schematic of the vacuum system (usually in section 7), so you can follow the bouncing air molecules yourself.
And, once you have all of this firmly understood, enjoy it briefly, it'll all be useless when airplanes all go electronic!