In a small pot on medium heat, add water, sugar, lemon zest, and thyme. Bring to a boil, then turn the heat down to a low and let simmer for 5 minutes. Take off heat and let cool completely. Once cooled, remove the thyme sprigs.
In a pitcher, pour in the simple syrup, limoncello, lemon juice, and vodka. Stir well, then add the sparkling water.
Serve over ice and garnish with a thyme sprig.