Stir together first 5 ingredients in a lightly greased roasting pan.
Bring brown sugar, butter, and corn syrup to a boil in a 3-quart saucepan over medium heat, stirring constantly; boil, without stirring, 5 minutes or until a candy thermometer registers 250°. Remove from heat; stir in vanilla and soda. Pour over popcorn mixture, and stir until coated.
Bake at 250° for 1 hour, stirring every 20 minutes. Cool in pan on a wire rack; break apart. Store in an airtight container.