Ex, whether direct manifold vacuum stops engine run on or not is debatable at best. I personally don't know since I have never had an engine so far out of tune that it needed such a tactic. The rest of your opinion on the matter is incorrect. The ported vacuum is metered by the position of the throttle plate and how much vacuum is available in the intake manifold and in my opinion you really don't understand the subject.
Cobra 29, here is a tech article that is helpful.
http://www.mustangmonthly.com/techar...on_tuning.html