To make the simple syrup: In a small pot on medium heat, add water, sugar, and rosemary. Bring to a boil, then turn the heat down to low and let simmer for 5 minutes. Take off heat and let cool completely.
In a pitcher, pour in the simple syrup, limoncello, the juice of 1 lemon, and vodka. Stir well, then add the sparkling water.
Serve over ice and garnish with a sprig of rosemary and lemon slices.