Around 20in vacuum at idle is fine...very good in fact. 18 is fine also. Did you check the 216 flywheel with top dead center of the 261? Idle speed ought to be around 450~550 with 500 being a nice medium. Sounds like yours is running too fast. At a certain rpm point you'll transition from your idle circuit to the "off-idle" circuit and it sounds like you're at that point.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end