It still sounds like a vacuum leak with it that low. You could do a compression test anyway, and if it is low you should probably have a leak down test done or do one yourself if youre set up for it. Its where compressed air is injected into each cylinder and the pressure observed to see if it `leaks down`. Its possible to tell by where the air leaks out, exhaust, the radiator etc, whats leaking in that cylinder. Intake or exhaust valves for example.
But if the compression was so low to cause that low of a vacuum, or the timing was far enough off to account for it, Im pretty sure it would present more obvious issues along with the low vacuum. Look real real close to be sure all the vacuum sources, manifold vacuum ones, are accounted for and block off as many things as you can. Check the manifold vacuum again. Maybe spray some carb cleaner or wd 40 around the base of the carb, carefully. See if you hear any difference in t he idle. The bolts that hold the carb on do get loose sometimes.