Preheat oven to 300ยบ. Place popped corn in oven while preparing caramel glaze. In medium saucepan, combine brown sugar, corn syrup, margarine, and salt. Cook ten minutes. Stirring constantly, bring to a boil over medium heat. Without stirring, boil four minutes. Remove from heat; stir in baking soda and vanilla extract. Pour over warm popcorn; tossing to coat completely. Bake 60 minutes, stirring every 15 minutes. Turn popcorn mixture onto a large piece of foil. Cool completely. Break mixture into small pieces. Place in large bowl.