Will a cloud form in a depressurized room? If a room is depressurized will a visible cloud or mist form?
I would expect it would, but when I apply a vacuum to a flask I see no cloud form inside the flask. Would a room behave differently than the flask?
I am more interested in the case of sudden evacuation than a gradual depressurization.
 A: The air in the room must be sufficiently laden (ideally, saturated) with water vapor (or another substance, such as alcohol, that can be absorbed by the air).  Then, if you drop the pressure suddenly, so that the air can expand adiabatically, the temperature can drop below the dew point of the vapor, and the vapor has an opportunity to condense into a cloud.  (The condensation is more likely in the presence of a little dust or some other points of nucleation; in perfectly clean, smooth conditions, I believe it's possible for the air to just become supersaturated.)
One way to do this is to hold the saturated air in one chamber while evacuating a neighboring chamber.  After the second chamber has been pumped out, quickly open a connection between the chambers by opening a valve or removing a partition.  Another way is to have the saturated air in a chamber fitted with a diaphragm, and suddenly to tug on it, expanding the volume of the chamber.  That's the approach used by Wilson in his cloud chamber, invented c. 1911 and for which he won the Nobel Prize in 1927; see:
http://en.m.wikipedia.org/wiki/Cloud_chamber
(Millikan was originally trying to perfect this technique to measure the elementary charge by studying the surface of a cloud in an electric field, but he, Wilson, and others were frustrated by complicating effects like re-evaporation of the cloud and its unsteady surface.  That's when Millikan discovered that with a large enough electric field he could actually disperse the cloud, leaving behind a small number of charged droplets suspended in the field.  If I recall correctly, Millikan's technique for dropping the pressure and producing the cloud was to open a valve to a connected low-pressure chamber.)
A: When you reduce the pressure in a vessel by pumping air out you reduce the temperature of the air. If the temperature falls below the dew point then water will condense out of the air and a mist will form.
The trouble is that the dew point depends on the pressure, and it falls as the pressure reduces. So whether a mist forms or not is dependant of how close you are to the dew point when you start pumping. If the air in your vessel is already close to satured with water a mist is likely to form, but if the air is fairly dry a mist is unlikely to form. It also takes time for a mist to nucleate and the droplets grow to a visible size. If you pump too rapidly it's likely you will remove all the air and water vapour before a visible mist has time to form.
You might want to try putting a small amount of water in your flask and leaving it a few minutes for the water to evaporate into the air in the flask. Then try pumping out the flask slowly.
A: If you have a syringe, close the outlet with your finger, push the plunger down hard, then let it go. A cloud will form inside the syringe, & will come out like smoke as you push the plunger back down. I discovered this when I was a kid. It didn't make sense to me at the time, why the fog occurred when it was depressurised rather than when pressurised, because I had seen somebody bleeding water out of a compressor tank, and I had heard that water boils at a lower temperature in low pressure & therefor, should evaporate more, and form a fog when pressurised.
