When you mix 7UP diet soda with vodka, the majority of the calories come from alcohol. Although the label on diet 7UP does not reflect a calorie content, when more than one serving is consumed, it does contain calories.
Aspartame is the artificial sweetener in diet 7UP. According to the Mayo Clinic, aspartame contains almost no calories. A full serving of 7UP diet soda is 8 oz.
A 1-oz. serving of vodka contains 65 calories. In most establishments that sell alcoholic beverages, a single shot of alcohol measures 1.5 oz, according to The Cool Spot.
The U.S. Food and Drug administration allows food with less than five calories to list zero calories on the nutrition label. Although diet 7UP says it contains no calories, it likely contains up to five calories.