It's not a dumb question. I don't think it can be done with a Mityvac, but there is a machine that uses pressurized air to suck out the coolant. I have seen it in use at Enrique's shop. He placed the conical rubber end of the suction hose into the expansion tank filler neck and opened a valve on the machine. In second it sucked every drop of coolant into the clear plastic container of the machine. I was totally mystified because it made no logical sense. He didn't touch anything else on the car.

I would consider buying that machine if I know where to get it - I don't have the nerve to ask Enrique where he got his. I have six cars to maintain, and it would really save time. On a Google Products search you can find similar machines that claim to do the same, but without an absolute guarantee that it would work like Enrique's machine I wouldn't be interested.