One problem I have seen with using manifold vacuum is that get into a loop where

1-RPM fluctuates and affects vacuum
2-which affects the ignition timing at idle
3-which affects the RPM,
4-go back to 1.

Ported vacuum works just like manifold vacuum to give you the same vacuum timing affect during cruise, but "shuts off" at idle.

One other thing, if you do have vacuum at the ported vacuum barb at idle you can be reasonably sure your idle screw is opened too far and you are going to have other drivability/tunability problems.