Scientificly speaking, 21" of mercury is a perfect vacume. It doesn't get any better than that.....even in space! LOL What you're seeing is a ported vacume signal from the carb, designed to be 0" at idle because the throttle plates are below the port in the venturi. As soon as the throttle plates start to move and there is air movement past the port it sends a vacume signal to the VA @ the dist.. With the high vacume readings that you've posted the VA should be all in by then, and if I remember correctly it takes in the neighborhood of 8" or 10" (depending on the VA/year of the dist/year motor/emission parameters) to pull the VA all in. High vacume readings also are the result of alot of timing advance at idle (maybe too much). Are you experiencing detonation at those lower RPM's? If not then all is well with the timing, if you are then back the inital back to the stock setting with the VA line disconnected and pluged......bottom line......you've got great vacume and that tells me that the rest of the motor is probably in great shape too. High vacume means that you should be experiencing crisp/responsive throttle response. Hope that helps.
Rob