In a small pot, combine the sugar and 1 cup of water. Bring to a simmer over medium heat, and simmer until the sugar is dissolved. Remove from heat and let cool completely.
Add the cooled simple syrup, sparkling water, lemon juice, and vodka. Stir to combine. Add ice and stir again. Cut the remaining peach into thin slices. Stir a few of the slices into the lemonade and use the others to garnish the glasses.