Preheat oven to 250 degrees F (95 degrees C). Place popcorn in 2 large shallow baking dishes or 1 large roasting pan.
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.
Bake in preheated oven, stirring every 15 minutes, for 1 hour. Remove from oven and let cool completely before breaking into pieces.