Any liquid whatsoever in the airlock will prevent outside air from getting into the fermenter. Water, vodka, whatever. It all works fine for this purpose.
Fungal spores and bacteria naturally drifting in air will find their way into the airlock (with bacteria, they hitchhike on small particulates, like dust particles). These will not grow and cause problems provided there is no carbon and other nutrients in the airlock liquid, because microbes need more than just water to grow. However, if you have ever had an explosive krausen that pushed through the airlock, there could be trace amounts of nutrients on the plastic that could support some growth. You'd want to clean that airlock very thoroughly.
Vodka is 40% alcohol, which is not high enough to be considered a true disinfectant. However, it does have some capacity to either suppress growth or kill cells, and so it's not totally useless as a disinfectant. Alcohol itself can be used as a carbon source by some microbes, but at 40% concentration, nothing is going to actively grow. Alcohol is maximally disinfecting at 70%.
I use vodka because I use plastic fermenters. No matter how carefully I try to move them, the plastic flexes and some liquid gets sucked in. Using vodka gives me some piece of mind. If I used a rigid-walled fermenter, I would just use water.