Is my Vacuum too low?

You can set the idle speed as low as you want to,until it stalls going into gear, or you end up with oil-pressure issues. The factory numbers are for emissions readings. If she bangs when going into gear, the idle-speed is too high.
Typically, the Rs will drop 50 to 100 rpm from neutral to engaged, and typically a teener will idle at 600/550 in gear with the anemic factory timing. So with 12* it could idle even slower. So in your case, the idle speed is sortof a personal preference. But if the Transfer slot exposure gets too far out of whack, you will have driveability issues such as bogs, hesitations,and soot. With a stock cam, she should have no problem idling down to 550 in gear. If it won't, then suspect a low-speed circuit issue. And with the falling out mixture screws , Ima thinking this.