Mix equal parts of water and simple syrup in a small saucepan to make the melon liqueur simple syrup. Over medium-high heat, bring the mixture to a low boil and whisk until the sugar dissolves completely. Remove the pan from direct heat and let the syrup cool and thicken. Pour into a mason jar, secure the lid, and cool in the fridge. When the syrup is cool, remove the cover, add one cup of melon liqueur, and mix thoroughly. When stored in a sterile air-tight container, the mixture will last up to six (6) months.
To prepare the cocktail, muddle the watermelon chunks in a cocktail shaker until they are broken down into small pieces. Add the vodka, lime juice, and melon liqueur simple syrup to the shaker, and fill it with ice. Shake the ingredients or stir using a long-handled bar spoon vigorously for about 30 seconds until the mixture is well chilled.
Strain the cocktail into a glass filled with ice, and garnish with a small wedge of watermelon and a wedge of fresh lime. Cheers!