Pour 2 1/2 cups grapefruit juice into 2 ice cube trays; freeze until firm. Cover and chill remaining juice.
Stir together water, sugar, and 2 rosemary sprigs in a saucepan; bring to a boil. Cover, reduce heat, and simmer 10 minutes. Remove from heat; discard rosemary sprigs. Cool syrup; chill.
Process frozen juice cubes, remaining 1/2 cup grapefruit juice, rosemary syrup, vodka, if desired, and chopped rosemary in a 5-cup blender for 10 seconds or until slushy. Serve in sugar-rimmed glasses, and garnish, if desired.
Fix it Faster: Use 3 cups bottled ruby red grapefruit juice.
Note: For sugared rims, dip rims of stemmed glasses into a thin coating of light corn syrup or water, and then spin rims in a plateful of sparkling white sugar.