In a blender combine the grapefruit and sugar; cover and blend until smooth. Strain through a fine mesh sieve. Freeze mixture in a 1 1/2- to 2-quart ice cream freezer according to manufacturer's directions. Transfer to a freezer container. Cover and freeze at least 2 hours. Serve sorbet with scoops of vanilla ice cream and Prosecco. Top with peel, if desired.