Just a guess at your set-up, but ported vacuum wont cause idle speed increase at idle rpm. However, an uncapped manifold vacuum tap will increase rpm, and once idle speed has reached sufficient rpm to activate mechanical advance rpm increases still more. Stock tune mechanical advance probably starts to kick in just below 1000 rpm.
Vacuum advance Can has to be connected to ported vacuum, a tap located above throttle plate that only sees vacuum once throttle plate is open so it will not advance timing at curb idle rpm.