Place the popped popcorn into two large baking pans.Preheat the oven to 200 degrees.Combine the brown sugar, corn syrup, butter and salt in a saucepan.Bring to a boil over medium heat, stirring enough to blend.Once the mixture begins to boil, boil for 3-4 minutes while stirring constantly.Remove from the heat, and stir in the baking soda and vanilla.The mixture bubble rapidly.Immediately pour over the popcorn in the pans, and stir to coat.It is okay if the popcorn isn't completely coated yet the oven will help us out.Bake for 45 minutes but stir the popcorn every fifteen minutes.When it comes out of the oven, try to separate the pieces as much as you can before it cools completely. Enjoy!