In a saucepan, bring to a boil the sugar, water, and vanilla bean. Once it boils add the frozen peaches and lemon juice and let them sit in the hot syrup 5 minutes. Remove the vanilla bean and puree the liquid. Strain into a shallow pan and freeze overnight. Scrape the next day and store in a container in the freezer until ready to serve the floats.
To serve the floats, place a spoonful of the scraped peach granita in a small juice glass. Top with 6 or so fresh blueberries. Pour tonic water over to cover and serve with a straw, cut the straw shorter, if needed, to fit the glass.