Preheat the oven to 250°F (120°C). Line two large rimmed baking sheets with parchment paper.
Combine the cereal and pecans in a large heatproof mixing bowl.
In a medium saucepan, combine the butter, brown sugar, corn syrup, and salt. Cook over medium heat, stirring until the butter melts and the mixture begins to boil.
Boil for 2 minutes, stirring gently. Remove from the heat and carefully stir in the vanilla and baking soda. The caramel will foam.
Immediately pour the caramel over the cereal mixture. Gently fold until evenly coated.
Divide between the baking sheets and spread into even layers. Bake for 1 hour, gently stirring every 15 minutes.
Let cool completely on the baking sheets, then break into bite-size clusters.
Notes
Cool completely before judging the crunch. The mix crisps up as it cools. Store in an airtight container at room temperature for up to 2 weeks.