Combine popcorn and pretzels in a large roasting pan; set aside. Combine brown sugar, butter, corn syrup and salt in a heavy medium saucepan. Cook over medium heat 12 to 14 minutes, stirring occasionally, until mixture comes to a full boil.
Continue cooking and stirring 4 to 6 minutes, until mixture reaches the soft-ball stage, or 234 to 243 degrees on a candy thermometer. Remove from heat, stir in baking soda and vanilla.
Pour over popcorn and pretzels in pan; add peanuts. Stir until mixture is coated well. Bake at 200° for 20 minutes; stir. Bake for an additional 25 minutes. Immediately spoon onto wax paper, let cool completely. Break into pieces; store in an airtight container. Makes about 24 cups.