(Part 2) Reddit mentions: The best pressure & vacuum measurement tools

Have you looked at something like this?

https://smile.amazon.com/gp/product/B01MCVOZJO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I'm experimenting with one to detect the differential pressure on my septic air pump to have an alert if it stops

Throw a ss pressure gauge in the one not being used! May need a few fittings or just throw a ball valve with pressure gauge attached to it because you will need a ball valve on it anyway....
http://www.amazon.com/Ashcroft-Duralife-Stainless-Connection-60/dp/B00FJNLRJG/ref=sr_1_16?ie=UTF8&qid=1422045666&sr=8-16&keywords=stainless+vac+gauge

Cheaper ones are out there for just psi...trying to find one for vacuum and psi at a better price...

Pressure sensor
https://www.amazon.com/dp/B07MHTWR1C/ref=cm_sw_r_cp_apa_i_7dTVDbYG37Y0N


Insteon sensor (only works with Insteon systems)
https://www.amazon.com/dp/B00O0LH9SQ/ref=cm_sw_r_cp_apa_i_bfTVDb50NSJS8


Zwave sensor

Looks like based on the photos in the description, this one may have screw terminals (the green square with screw heads on the photo of internals)
I don't know for sure so you'll need to do your research.

https://www.amazon.com/dp/B07PDDX3K6/ref=cm_sw_r_cp_apa_i_ZjTVDbJC5H32A

There's not a lot to show, but I threw together a couple of quick wiring diagrams:
https://imgur.com/gallery/aKN9joL

Otherwise, here are the links I posted above:
ADC board: WaveShare AD/DA board based on the ADS1256
Diff. Pressure sensor: DIYmore Breakout Board for the MPXV7002 sensor

And here is a PDF of the actual schematic of the WaveShare board as posted from the WaveShare AD/DA page posted prior:
https://www.waveshare.com/w/upload/2/29/High-Precision-AD-DA-board.pdf

That should probably be fine. I might suggest a somewhat weak test of putting a slight kink in the hose pointing upwards... if the vacuum is struggling against it, it'll suck the hose back to straight.

Sadly, my testing methodology isn't particularly portable. I was given an 11 (?) inch absolute pressure gauge (Heise; this style) from the '60's, which I have up on display on the wall, but for fun is plumbed to a pneumatic quick connect. So I basically just patched it over to the vacuum using some pneumatic hose. A gauge like this could do the job, but TBH I don't think it's worth it unless you have another use for a vacuum gauge.

The connection part was stupid simple though: cut off a few inches of random 1/4" hard plastic tubing, drill 1/4" hole in the F-F coupling connector, jam the tubing in the hole, apply hot glue to prevent leaks. From there, it was just whatever connectors were handy. (I have a set of vacuum hold-down clamps, which helps explain why this array of stuff was readily available)

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As a significantly less scientific approach, you could connect a host straight to the vacuum, use a blast gate or even just a hand to block the flow varying amounts, and note how the sound changes. Then you can put the same hose setup as you normally use, just without the separator, and see how it sounds. Finally, with the whole setup, again. That should at least give a rough estimate for how much work and flow restriction it's all causing, and establish if it's due to the hoses, or due to the separator.

E: Addendum: that assumes that you have no leaks, and all airflow loss is due to pressure drop. If it doesn't sound like the vacuum is working any harder, but you get noticeably less airflow, then you likely are in the "leaking somewhere" regime. I don't think that's likely though, because you need a pretty enormous leak in order to appreciably affect the vacuuming performance of the Fein.