Cucumber juice and zingy lemongrass combine in this refreshing vodka sipper. To make cucumber juice with a blender, purée about 2¾ cups cubed cucumber until smooth, then strain through a fine mesh strainer.
Up to 2 hours prior, combine all ingredients except garnish with 2 cups water. Store in refrigerator.
To serve, stir mixture. Pour into punchbowl over large ice block. Insert basil leaves into cucumber slices, and float atop mixture with leaves upright. Serves 15–20.