I've never seen one but I remember old photos from the 70s of cats with a track on the deck/tramp that runs around behind the mast. So the vang just goes straight down to a car on the track. Very elaborate, very heavy and absolutely doesn't do the only thing a vang is actually useful for (rotate the mast).

I would say the rule (like a few others) is redundant these days.

I don't even have a vang on mine.