Grapefruit is prepped three ways for this citrusy version of the Spanish punch -- juiced, supremed, and thinly sliced into wheels.
Bring sugar and 1/3 cup grapefruit juice to a boil, stirring until sugar dissolves; let cool completely. In a pitcher, stir together syrup, remaining 3 cups grapefruit juice, vermouth, and grapefruit segments and wheels. Cover and refrigerate at least 1 hour and up to 1 day. Serve over ice; garnish with a grapefruit wheel.