Thanks everyone for your input.

Admittedly however, I still have not found the answers to my idling/vibration problem.

Frankly, this 91 740 does not have an idle speed adjustment anywhere to be found! I have ultimately come to the following conclusions:
1.Most of the vibration problem would be cured if I can raise idle a few hundred rpms to about 800rpm.
2.Engine mounts do need replacement however.
3.Checked vacuum leaks and everything seems intact, replaced one hose and cleaned alot of stuff.

I need help on one specific question: On the throttle body housing plastic connector, before the accordion plastic duct which connects to the air housing, there are 2 small air holes, one is connected to a hose, the other one is simply open and exposed to the outside air! Is this normal? It doesn't seem so, but I can't figure it out!

Thanks again.