Prepare two large baking sheets by lining them with parchment paper and set aside.
Preheat oven to 250 degrees F.
Add popped popcorn to a very large bowl.
In a medium saucepan over medium heat, melt the butter. Stir in brown sugar, corn syrup, and salt. Bring the mixture to a boil, stirring constantly. Boil without stirring 4 minutes. Remove from heat and stir in soda and vanilla extract.
Pour over the popcorn, and stir to coat.
Distribute the caramel in one layer onto the prepared baking sheets and bake stirring every 15 minutes, for 1 hour. Remove from oven and let cool completely before breaking into smaller pieces.