Place 2 martini glasses in the freezer for 10 minutes or longer. Fill a cocktail shaker halfway with ice. Add 3 ½ ounces lemon-scented vodka, 2 tablespoons fresh lime juice, 2 ounces Cointreau and 1 tablespoon fresh cranberry juice; cover. Shake vigorously for 30 seconds. Strain evenly into the chilled glasses. Top with lime zest.