First off my practical experience of refrigeration is limited, but I have used vacuum ovens a lot. These always had a nitrogen bleed at the opposite end of the chamber to the vacuum connection, say 20ml/min in an oven of about a cubic metre. Without this, solvents with higher boiling points (60-80C) would not leave the oven, they'd evaporate off the stuff you were drying (oven set at 40C) and pool in the bottom. A small gas bleed carried away the solvent vapour to the vac pump, prevented the oven being a dead end. I've no doubt Argus is right (and you others), but I would have though that filling a system with nitrogen and sucking it back out will better help shift moisture. But then maybe you only need to do it once after all?