Combine first 4 ingredients in a small saucepan. Bring to a boil over high heat; cook, uncovered, 8 to 10 minutes until sauce is reduced to 1/3 cup. Remove from heat. Place raspberries in a medium bowl. Pour sauce through a wire-mesh strainer over berries; let stand, uncovered, 30 minutes or until cool. Stir in vanilla.
To serve, place 3/4 cup nectarine slices in each of 4 dessert dishes; spoon 1/4 cup raspberry mixture over each serving.
Macerating is similar to marinating. A food, usually fruit, is soaked in a liquid such as wine, liquor, or a flavorful syrup to infuse that food with more flavor.