I vacuum pack mice and rats for later feedings, but only after they are already dead, and frozen. Vacuum sealing a live rodent would certainly kill it, but...dang! That's harsh. Even with the whole "is it humane" issue set aside, there is at least one more thing to consider. It could get messy. I have learned to freeze mine (for at least a couple of hours) before I seal them just for that reason. Even the home-use vacuum sealers can turn a pinky into jelly. It wouldn't be AS bad for full grown mice or rats, but I suspect it would still be unpleasant.
Killing by C02 doesn't have to be expensive. If you kill several at one time or even within a day or so, Dry ice and water are pretty easy to come by in most areas and not too expensive. I say 'within a day or so' because dry ice won't last much longer than that in your freezer, and you usually buy it by the pound. It becomes cost-prohibitive if you are buying a pound of dry ice to kill 2 rats. I have heard that some people generate C02 by mixing vinegar and baking soda (a la science-fair volcanoe). I actually attempted this a few times when I first started breeding. I wasn't too successful. It took a whole lot of both ingregients to produce enough CO2, I made an enormous mess, and the rodents ended up smelling like pickles.
One thing that DID work pretty well for small "batches" was the little bb-gun CO2 cylinders. In the camping gear department of your local Targetwalmartwhatever, look for a camp-stove pressurizer. Someone apparently go tired of having to pump air into his camp stovelantern to pressurize it every 15 minutes. So they came up with this little device that screws on instead of the cap. You pop a CO2 canister in it, turn the knob and the gas is released. is you have a small number of rodents and a fairly small container, this does work. You may also visit the "head-shop" in your town (don't lie, you know where it is). They often sell "crackers", or "poppers" or something like that that people use to open nitrous oxide containers (similar in size and shape to Co2). My store was out of them when I asked. Must be pretty popular.
Disclaimer: Crotus does not believe that inhaling anything other than air (and occasionally helium to sound funny) is very bright. Those people who do so to "catch a buzz" are stupid, and deserve what they get.