Vodka over beer lasts till bottling day just fine for me. Over some of my wines I might have to top off the air lock but I don't mind. Keeping an eye on things is part of home brewing.
So far so good. My airlock actually exploded from all the action. I think in the future I will be using either water or vodka in my airlock.
Cheap vodka is def better than plain water. Nasties can grow in plain water.