In a medium saucepan, combine the butter, brown sugar, and corn syrup and cook over medium heat, stirring frequently, until it begins to boil. Allow to boil for 2 minutes (3 if you’re at a high elevation) and then remove from heat. Add the vanilla and stir until combined. Drizzle the caramel mixture over the cereal/nut mixture and stir quickly to combine. Transfer to the prepared baking sheet. When the caramel mixture has cooled but isn’t solid yet (just a few minutes), add the Pretzel M&Ms and toss gently to combine. Allow to cool completely before serving.