Preheat the oven to 300 degrees F. Line 2 baking sheets with foil.
In a large bowl, combine the popcorn, cereal, pretzels and peanuts. In a medium saucepan, combine the brown sugar, butter and corn syrup. Cook over medium heat, stirring, until the mixture comes to a boil. Reduce the heat to medium low and simmer without stirring for 5 minutes. Remove the saucepan from the heat and carefully stir in the vanilla and baking soda. Pour over the popcorn mixture and stir until coated.
Divide the mixture between the prepared baking sheets and bake, stirring every 10 minutes, for 30 minutes. Transfer the pans to wire racks and let cool completely, continuing to stir occasionally as the snack mix cools.