Losing Vacuum Pressure in '95 E420???

When I drive the car under moderate acceleration and come to a stop light, the tach is around 550 RPMs and holds there for a moment and then slowly settles to a touch above 500 RPMs. Takes about 5 seconds.

If drive like a grandma, the tach is just around a tad above 500 RPMs when the car comes to a stop.

At the 500 RPM range, the car has a slight shake felt through the seats and steering wheel (which I've complained about in other posts). Now I'm curious as to why the idle settles from 550 down to 500.

Is the computer controlling this, or is the car losing pressure somewhere? Does vacuum pressure play a roll in the idle speed? Is there something I can check, adjust or play with to keep the idle at 550 RPMs???

Thanks.