Originally Posted by S-Class Guru

Yeah, about 15-16 has been normal on mine for the last 10 years or so.
This puts the needle on the left peg when idling in neutralat normal RPM of about 700, with warm 10-40 oil. When in gear the needle will move off the peg about 1/8th inch.

I assume the engine runs smooth at idle and speed, just sluggish? If the engine has a problem and is struggling a bit, then the IAC will open more to maintain idle speed, and the vacuum will fall a bit; so that may be what you are seeing. So, the lower vacuum would be a sympton, not a cause.

You might check compression, cam lobe wear, and possibly cam timing (although the timing chains are pretty bulletproof).
A quick check of the fuel system can be done by checking the Lambda percentage as defined in some of the archives herein.

DG