In a blender container or food-processor bowl, place the undrained pineapple. Cover and blend or process until pureed. Transfer the pureed pineapple to a mixing bowl; stir in sugar. Let the mixture stand about 10 minutes or until sugar dissolves, stirring occasionally. Stir in buttermilk and vanilla.
Transfer pineapple mixture to a 1-1/2-quart freezer container. Cover; freeze overnight or until firm. Let stand at room temperature about 30 minutes before serving. Scoop into small dessert bowls. Serves 8 or 9, 1/2-cup servings.