Combine vodka, lime juice, and cranberry juice in a pitcher with 2 cups water and stir to blend. Divide between ice cube trays and freeze at least 4 hours or overnight (the drink cubes will freeze only partially).
Just before serving, empty trays into a large bowl and using a fork, break up cubes to loosen ice crystals (the ice will be slushlike). Spoon slush into tall stemmed glasses and serve immediately.
