Juice the grapefruits and divide the juice among 4 glasses. Add 1 1/2 ounces of vodka to each glass. Add desired amount of simple syrup to sweeten the drink. Stir well.
Dip the grapefruit slices in sugar, to coat. Place 1 sugar dipped grapefruit slice on the rim of each glass.
Combine the water and sugar in a saucepan over medium heat. Bring the water just to a boil and boil until the sugar dissolves, but the mixture does not get any color. Remove from the heat and let cool. Store in the refrigerator for up to 1 month.