Vodka is pure ethanol which is an efficient carbon source for waterborne bacteria. When more carbon is available,more bacteia grow taking up dissolved nutirent material into their bodies. The bacteria are inturn taken out by the skimmer more readlily than the dissoved organic material . They may also be consumed by som,e corals.The result of this is a reduction in dissolved oraganic material which nuisance algae (hair algae,diatoms) need. It may be that xenia need this material as well.
Oxygen is not really an issue unless the bacteria crash.