Simple Syrup: In a small sauce pan over medium low heat, dissolve the sugar into the water for about 5 minutes. Do not let the mixture boil or crystals will form. In a heat safe container, pour the warm sugar water mixture over the rosemary sprigs and let it steep for at least 1 hour. The rosemary simple syrup will keep in the refrigerator in an airtight container for 10 days.
Cocktail: In tall glass filled with ice combine the vodka, rosemary simple syrup and grapefruit juice. Garnish with a sprig of rosemary.