For what it is worth, there are 2 different ways of looking at this. What will kill the criters, and what will stop them from growing. I am not aware of any yeast that can ferment to 40%, so vodka will stop any new growth, and extended exposure times will (almost) always make up for lack of concentration of the agent.
I put the vodka in the air locks, but I would not use it as a sanitizer.
Sorry for jumping in late,
Wonderful 5 page outline.
A good description
of alcohol directly related to brewing is here. The bold section was my doing.
The most commonly available alcohols that can be used for sanitizing are methyl, ethyl, and isopropyl. Alcohol's mechanism of action is still unconfirmed, but theories for how alcohol might kill cells include denaturing of cell proteins, interfering with cellular metabolism and destroying cell membranes. In the absence of water, proteins are not denatured as readily by alcohol, and this explains why a solution of 70 percent alcohol and 30 percent water is a better sanitizer than 100 percent alcohol.
Alcohol will kill most bacterial organisms in less than five minutes, but because some organisms may take longer, it is best to let items soak at least 10 minutes to kill the majority present. Alcohol does not kill bacterial spores, and viruses are only killed after exposure of an hour or more, but these microorganisms are not a concern to brewers. As with all sanitizers, the degree of effectiveness is dependent on the initial cleanliness of the surface.
Alcohol as a sanitizer has limited uses in brewing. A major limitation is that all types of alcohol are reasonably flammable even at a 70 percent solution. Isopropyl and methyl alcohol are much more toxic if consumed than is ethyl alcohol, and are undesirable in finished beer because of this, let alone their undesirable flavor. Isopropyl alcohol is the most effective sanitizer of the commonly available alcohols, with ethyl alcohol being a close second. Methyl alcohol is not a very effective agent compared to the other two and this fact, combined with its toxicity, means it is not often used as a sanitizing agent (4,5). For these reasons, ethyl alcohol is the more favored alcohol for sanitization, but is rather expensive because concentrated forms are highly taxed.
Alcohol is useful for sanitizing equipment and surfaces used in yeast culturing and propagation. Isopropyl alcohol at a concentration of 70 percent is an excellent, inexpensive choice for sanitizing work surfaces, bottle and flask necks, instruments and your hands. The alcohol can be applied to surfaces in a number of ways, the easiest being with a small spray bottle. A piece of gauze or cotton soaked in alcohol can be used to wipe down surfaces such as tables and container openings, or instruments can be soaked in alcohol until needed. Alcohol such as isopropyl and ethyl are safe to use on most surfaces. Don't use alcohol to sanitize tubing because it can dissolve the plastic to some degree. Some plastics, such as HDPE, are generally resistant to alcohol. Metals and glass also are unaffected.
It is often stated in homebrewing lore that you can simply gargle with vodka or some other high-proof alcoholic beverage and then use your mouth to start a siphon without fear of contamination. But based on the effectiveness of alcohol, this does not seem to be such a wise idea. First of all, alcohol's ability to kill bacteria, i.e., denature proteins, is constrained by the total amount of organic material present, which for the average mouth is a fair amount depending on when the last meal was consumed. Second, an 80-proof beverage such as vodka is only 40 percent alcohol and most organisms are not killed in less than five minutes at this concentration. For this method to be effective, you would have to gargle with 120 proof rum or something of equal strength for 10 to 15 minutes, by which time you probably would have forgotten about brewing. Rather than risk contamination, use a small tube that fits into the end of the racking hose and suck on that to start the siphon. Once the siphon starts, remove the small piece of tubing before the wort reaches it and you don't risk contamination.