The only vacuum retard I've ever seen was on a late-60's Mustang with a 200 cubic inch 6-cylinder. It had a 2-chamber vacuum system that retarded the spark at idle to reduce hydrocarbons, and then provided a 10-degree increase over the centrifugal advance at light-throttle cruise. Anytime the throttle is opened to provide more power under load the spark is retarded, that's true, but at cruise, there's a considerable amount of advance over centrifugal for gas mileage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!