Pre-heat oven to 250°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 pre-heated oven, stirring every 15 minutes, for 1 hour.
Remove from oven and let cool completely before breaking into pieces.
