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 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.

Remove from oven and let cool completely before breaking into pieces.