Sorry, heading out on vacation today and no time to take pics/download etc. In the lid of my buckets, the hole is about an inch wide for a rubber stopper. The vial fits in where the stopper would go. It's not an airlock, just a stopper. The fit is pretty tight.
I'm not worried about oxidation because I only leave my beers in primary for about 2 weeks before kegging. Half the time, the airlock on my buckets doesn't even bubble because there is so much CO2 that escapes around the rim of the bucket from the poor seal on the lids. I wouldn't shove the vial into the hole if I thought that there was no other way for the gas to escape but it can easily get out through the bad seal. That's also why I wouldn't reverse the process. If the release of gas was too slow, it would blow the vial out of the hole. Maybe there is a chance for oxidation but I don't believe it's any worse than with an airlock in this case. I also know there are brewers out there that just put foil over the mouth of their carboys, occasionally myself included who don't have problems.
It was just a make-shift solution to a problem I discovered I had (too few airlocks). I keep buying them and either losing them or more often, breaking them (I'm very clumsy).