Preheat oven to 250°F. Place popcorn in a very large bowl. Grease two large, shallow baking dishes and set aside.
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 baking soda and vanilla. Pour in a thin stream over popcorn, stirring to coat.
Place in prepared baking dishes. Bake in preheated oven for 1 hour, stirring every 15 minutes. Remove from oven and let cool completely before breaking into pieces.