vacuum advance idle issue?

If the engine rattles, the timing is too far advanced. Make sure your vacuum advance is attached to ported vacuum. That means at idle, there is no vacuum applied. As the engine RPM increases, vacuum is applied. There should be a vacuum port ABOVE the throttle plate that is ported.

To set the timing, disconnect the vacuum advance from the distributor and plug the line. Get a timing light and set the timing to 10* before top dead center. Reconnect the line and drive it. Readjust as needed. What I would do is advance it until it rattles on a slight incline going slow lugging in high gear, then retard the timing until it stops and lock it down right there. Recheck it with the timing light and record where it is so at the next tune up you will know where it likes to be set.