Place the pineapple in a plastic container in the freezer until completely frozen, at least 3 hours.
In a blender, combine 1 1/2 cups spiked watermelon, 6 tablespoons frozen pineapple, and 1/2 cup Citrus-Ginger Infusion. (Amount of infusion may be increased or decreased according to individual taste.) Blend on high speed until well combined. Add enough ice to fill the blender, 3 cups, and additional vodka, as desired, and blend until smooth. Pour into tall chilled glasses or a pitcher. Repeat with the remaining ingredients, to make about 10 blenderfuls. Garnish with mint sprigs and serve with a long straw.
Yield: about 20 servings