Place the pretzels, popcorn and peanuts into a large paper bag or, if you have one, a very large bowl. Set aside.
In a saucepan over medium heat, combine the brown sugar, corn syrup, and butter. Cook, stirring to melt the butter and dissolve the sugar, until the mixture comes to a boil. Once the mixture is at a full boil, continue to cook for 5 minutes, stirring constantly.
Remove from the heat, and stir in the salt, vanilla, and baking soda. The mixture will foam up when you add the soda, so use a long spoon to stir. Immediately pour the sugar mixture over the popcorn and pretzels in the bag. Shake until evenly coated. For a crispier treat, boil the sugar mixture longer, or bake the finished mix in the oven at 350 degrees F (175 degrees C) for about 10 minutes.