Mix the melted butter into the finely ground vanilla wafers and add 2 Tablespoons of water. Mix well and press the crumbs onto the bottom of the prepared pan, forming a crust. Bake the crust for 20 minutes and allow to cool.

Set aside 2 cups of the watermelon juice in a small bowl. In a large bowl, combine the remaining 3 cups of watermelon juice with the lemon zest, lemon juice, sugar and milk.

Sprinkle the gelatin over the reserved 2 cups of watermelon juice. Allow to sit for 2 minutes so that gelatin can dissolve and begin to absorb liquid, or "bloom." After the gelatin blooms, heat it in the microwave for 2 minutes on high, or heat gently in a small pot over low heat on the stove top, until the gelatin has completely melted into the juice and the liquid is smooth.

Add the watermelon juice with the melted gelatin to the larger bowl of juice from Step 3. Stir to combine. Gently pour the mixture over the prepared crust from Step 2 and place in the freezer for 2 hours or until the mixture is firm. (Alternately, place the pan in the refrigerator and allow to set for 3 to 4 hours.)

Keep in the refrigerator until ready to serve. Before serving, cut the watermelon bars into squares. If desired, sprinkle each serving with confectioners' sugar or a dollop of whipped cream.