I went to my local Lowe's hardware and matched the Mityvac's nozzle diameter with a clear plastic tube that fit it perfectly. I can't remember the ID of that tube, but it was slightly larger than my racking cane tubing. I then got a tapered nylon pipe nipple also from Lowe's that fit snugly into the tubing, and jammed the other end of that into a stopper that is one half-size larger than that which I normally use to airlock a carboy. I went with the oversize stopper so there is no danger of the stopper being drawn too far down the carboy neck from the vacuum.
When I pump down a carboy to outgas the contents, I just squeese a few times on the trigger lever, to draw about 5 to 8 inches of mercury vacuum. This will start a slow bubbling up of CO2 from the bulk liquid, and you'll have to squeeze the lever regularly (about every 30 seconds or so) in order to keep the level of vacuum around that point. As I said earlier, if you pull much more vacuum than that, you'll get a LOT more CO2 coming out of solution much faster, but that may not be good for aromatics in the final mead.