It's really quite simple. The IACV regulates idle speed above the base idle set by the screw. When the coolant running thru it is cold, it idles faster, when the coolant is up to temp, it shuts off, and lets the screw determine the idle speed. Chances are, when you're setting your idle speed, your engine/coolant are not completely up to operating temperature. If you don't disconnect the IACV, it will throw off the base idle setting that you're trying to dial in. If you set the idle with the IACV disconnected, and then reconnect it, the idle speed will probably jump up a couple hundred RPM. Set the idle speed with the IACV disconnected, reconnect it and jump the ECM if necessary. When you start the car again, it'll seem to idle high because the engine is cold and the IACV is idling the engine faster to warm it up. Once it's warmed up, it'll shut off, and idle will drop to the speed set by the screw. A properly functioning tb and idle adjusting screw will not cause an external vacuum leak. I suggest you find the source of the leak before you try setting a consistent idle speed.