Preheat oven to 250 F. Spray two large rimmed baking sheets with non stick cooking spray. Place popcorn and peanuts in a very large metal 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 add in soda and vanilla, stir to combine. Immediately, pour sugar mixture in a thin stream over popcorn. Stir popcorn to thoroughly coat.
Spread popcorn into prepared baking sheets, and bake in preheated oven, stirring every 15 minutes, for 1 hour. Remove from oven and let cool completely before breaking into pieces.