Preheat oven to 250 degrees F. 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 over popcorn, stirring to coat.
Place caramel popcorn in two large shallow baking dishes and bake in preheated oven, stirring every 15 minutes, for 1 hour. Remove from oven and let cool completely then break into pieces.