The reason why a vodka soda may not leave us as dehydrated as other cocktails is actually a bit multi-faceted. For one, different types of alcohol contain varying degrees of complex organic molecules known as congeners, according to 2010 research published in Alcoholism, Clinical and Experimental Research. Acetone, acetaldehyde, and tannins are just a few of the different kinds of congeners found in most alcoholic drinks. In the study, researchers found a link between congener content and hangover severity in young adults, outlining thirst as one such hangover symptom. The study findings showed that participant hangover symptoms were more severe in relation to drinking bourbon over vodka, which is said to have 37 times more congeners than its clear counterpart.
The other, probably somewhat more obvious reason that a vodka soda is a less-dehydrating cocktail choice is because the beverage includes water, which may lessen rising blood alcohol levels (via Food & Wine).