I just went to the lawn mower section and bought a plain old fashioned fuel filter. I guessed at which direction to install - but - I bought the kind with a plastic mesh for the filtering media, not a paper element. I too was concerned about possible vacuum restriction. Vacuum readings were the same before and after - been at least a couple months with no issues

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'85 300D - 206K (sold)
'81 240D - 149K (sold)
'03 Jetta TDI - (sold)
'79 300D - (sold)

Yea, I put it in to let me know there is a problem before it actually does its damage to the vacuum system by polluting it with oil. It was a fuel filter off of a ealier (gas) Mercedes, I don't recall if I got the number from benz.org or here. I think I have a second filter around somewhere so I can get the number with a little effort.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I believe it is a filter used to prevent any foreign material from getting into the vacuum pump. It also gives a visual indication that something has come apart or there is a vacuum leak letting foreign material get into the system. Think about, we all use air filters to filter the air coming into the engine.
I really don't know, but this is food for thought.

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84 500 SEL (307,xxx miles)

I have the filters in my Vacuum lines and it saved my butt 2 weeks ago. Sucked transmission fluid up from a bad modulator. When the car was slow to shut down, I checked under the hood and found transmission fluid in the filter. But It didnt suck any into the AC control system or door locks. It was worth every penny of the $ 3.00 they cost. Replaced the modulator and filters and all was well.... Russ

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1978 300 D 258,000 + miles sold 1995
1983 300 D Turbo 152,000 miles
Thank God for the man who put the white lines on the highway