That link exlpians better what I was trying to say, it is additive at all rpm, but the vacuum is not adding any at low vacuum readings that occure at high rpm. It actually retards the timing at the higher rpm-the mechanical advance is all that is required. I checked the link with an engine performance manual and an ASE study guide-they both state that the vacuum advance primarily is to offset lean feul mixtures at low rpm/high engine load.
