Preheat the oven to 250°F and line a baking sheet with parchment paper. Place the popped popcorn in a large bowl and set it aside.
In a medium saucepan, melt the butter over medium heat. Stir in the brown sugar and corn syrup until the mixture comes to a boil. Let it boil for 4 minutes without stirring.
Remove the pan from the heat and stir in the vanilla extract and baking soda. The mixture will bubble up slightly.
Pour the caramel over the popcorn in the bowl and toss gently with a spatula until the popcorn is evenly coated.
Spread the coated popcorn onto the prepared baking sheet in a single layer. Bake for 20 minutes, stirring once halfway through.
Remove the baking sheet from the oven and let the popcorn cool for 10 minutes. Stir in the pretzel twists, candy corn, mini marshmallows, and Halloween sprinkles until evenly distributed.