Preheat oven to 250 degrees F (95 degrees C). Place popcorn in a very large bowl.
In a medium saucepan over medium heat, melt butter. Stir in brown sugar, corn syrup and salt. Bring to a boil, stirring constantly. Boil without stirring 4 minutes. Remove from heat and stir in soda and vanilla. Pour in a thin stream over popcorn, stirring to coat.
Place in two large shallow baking dishes and bake in preheated oven, stirring every 15 minutes, for 1 hour.