Preheat the oven to 250 degrees F (120 degrees C). Spray a large shallow roasting pan with cooking spray, then fill with popcorn.
Place in the preheated oven while preparing caramel.
Mix brown sugar, corn syrup, butter, and salt together in a heavy 2-quart saucepan over medium heat; stir constantly until mixture comes to a boil.
Stop stirring, and boil mixture for 5 minutes.
Remove from the heat and stir in baking soda and vanilla until well combined.
Pour syrup over warmed popcorn and stir until evenly coated.
Return to the oven and bake for 45 minutes, stirring occasionally.
Remove from the oven and spread onto a large foil sheet greased with cooking spray.
Cool, then break apart and store in a tightly covered container.