Gef is right. Regardless of whether you're drinking gin, vodka, whisky, Bacardi or other high alcohol spirits, they have virtually the same the same calorific value.
Low calorie vodka does not exist unless you're talking about the cheap pseudo-russian, �4.99, 30% gut-rot stuff available at your local late-night garage. But that's not real vodka is it? Remember, the lower the alcohol content, the less the calorific value.