Boil the vinegar in a small saucepan over medium-high heat until reduced by half, about 3 minutes. Cool slightly.
Toast the pine nuts in a small, dry skillet over medium heat, stirring often, until golden brown, about 3 minutes.
Whisk together the reduced vinegar, olive oil, 1/4 teaspoon salt and a few grinds of pepper.
Toss the cantaloupe, radicchio, mint and toasted pine nuts together in a wide, shallow bowl. Drizzle with the vinaigrette. Toss again right before serving.