Fill two large glasses with ice cream scoops. Divide the watermelon juice evenly between the two glasses and then pour the vodka in too. Gently stir a bit if you can--you don't want to totally melt the ice cream. Pour the soda over top of the juice until it reaches the top. Add some straws and sip sip!
To make the watermelon juice, puree watermelon cubes in a blender until they are liquid. Strain the mixture over a mesh sieve into a measuring cup. The amount of watermelon you need will differ depending on the water content of the fruit, but a baby seedless watermelon will yield a few cups.