Preheat oven to 400 degrees with rack in the center. In a 10-inch ovenproof skillet, combine 2 tablespoons oil and 2 teaspoons vinegar. Arrange radicchio in the skillet, placing 1 sprig of rosemary underneath and on top of each wedge. Drizzle remaining 1 tablespoon oil and 1 teaspoon vinegar over radicchio. Season with salt. Cover with plastic wrap, and then aluminum foil. Roast until tender and caramelized, 30 to 45 minutes.