Preheat the oven to 250F. Line 2 baking sheets with parchment paper.
Put popcorn in a large bowl.
Put butter, brown sugar, corn syrup and salt in a saucepan. Bring to a boil over medium heat. Reduce heat to simmer and cook, stirring constantly, for 5 minutes.
Remove the saucepan from heat, add baking soda and vanilla and stir to combine.
Pour the mixture over the popcorn immediately, and quickly toss the popcorn so all of it is coated with caramel.
Put the caramel popcorn on the parchment-paper-lined baking sheets in a single layer. Put in the oven and bake for 1 hour.