234 calories in 8 ounces! That's just 55% on the efficiency scale. Cranberry juice packs a serious caloric punch. Worse yet, cranberry "juice" taps at bars tend to be extra concentrated and light on the juice factor, so forget about any antioxidant benefits.
Sip this, ditch that: Vodka, Soda with a splash of cranberry. Not 100% efficiency, because you're probably getting about an ounce of cranberry if you order at a bar, but its efficiency is in the 90% range (saving about 80 calories) and it should taste reasonably good. Cranberry juice is an extremely potent mixer, even in small quantities.