Place popcorn in a large bowl and set aside. In a large saucepan, combine the brown sugar, butter and corn syrup; bring to boil over medium heat, stirring constantly.
Boil for 5 minutes, stirring occasionally. Remove from the heat. Stir in vanilla and baking soda; mix well. Pour over popcorn and mix until well coated. Pour into two greased 15-in. x 10-in. x 1-in. baking pans.
Bake, uncovered, at 250° for 1 hour, stirring every 15 minutes. Cool completely. Store in airtight container or plastic bag.