listening to an interview with John Palmer he suggests putting the fermentation vessel in the fridge (after fermentation is complete) to drop out some left over trub and yeast. I figured I would give it a shot, but when putting it in I wondered what would happen to the air-lock when the beer cools, it would surely start sucking in instead of blowing out. what other methods of keeping the atmosphere and other stuff out could I use, if I just put a stopper on top how would a vacuum effect the beer?
I learned this the hard way, and you are correct; the cooling beer will create a vacuum and suck the air lock fluid into the carboy. What I do is put either a solid bung or a carboy cap on the carboy before crash cooling. I kind of prefer the cap because the vacuum won't pull it into the neck of the carboy, whereas there's a chance that could happen with a solid stopper.
Crash cooling is a great idea to help clarify your beer if you're going to keg it or otherwise don't need the yeast for conditioning. I do it with every beer. I wait about four days after I'm sure fermentation is done, then I move the carboy to the beer fridge for three days before racking to the keg.
Since fermentation is done, you don't need an airlock at all. I use a stopper to seal buckets, or some plastic wrap and a rubber band to seal carboys. Don't over think this! And even though the yeast will tend to drop out, there will still be plenty left for bottle conditioning. I've had no trouble bottling beers that have been lagering for months.
Just fill your airlock with something that living microbes can't survive in, like vodka or sulfite solution and don't fill it up so high that any will be pulled into the carboy by negative pressure. And RDWHAHB about the scary oxygen that will be pulled in, it's negligible and also it will stay above the cushion of CO2 that already in the neck of the carboy.