Cut the watermelon into small chunks and blend until smooth.
Strain the watermelon puree through a fine-mesh strainer to remove pulp.
Fill a cocktail shaker with ice.
Add vodka, watermelon juice, lime juice, and simple syrup to the shaker.
Shake vigorously for 15-20 seconds until well chilled.
Fill a highball glass with fresh ice.
Strain the mixture into the glass.
Top with a splash of club soda.
Garnish with a watermelon wedge and mint sprig.