Combine popped corn and pecans; keep warm in 225 degree oven. In saucepan, combine sugar, butter and corn syrup. Bring to boil over medium heat, stirring constantly. Cook and stir until mixture turns caramel color (about 280 degrees on candy thermometer). Remove from heat; stir in vanilla. Place popped corn mixture in large greased pan; carefully pour sugar mixture over. Mix well; turn out. Quickly break into cluster. Store in tightly covered container. Makes about 9 cups.