1/4 cup simple syrup (equal parts sugar and water, heated until sugar dissolves; cool)
1/4 cup fresh lime juice
Pour cranberry juice into 3 ice cube trays, place in the freezer and freezer until firm.
Place vodka, simple syrup and lime juice in a blender and blend until combined. Fill the blender with cranberry ice cubes and blend until a thick slushy consistency forms. Pour into frozen mugs and serve with a straw.