Preheat oven to 250ºF.
In a large bowl, combine cereal and pecans.
In a large saucepan over medium high heat, combine brown sugar, corn syrup and butter. Bring to a boil, stirring occasionally. Remove from heat and stir in vanilla and baking soda. Pour over cereal mixture, tossing to coat evenly.
Pour cereal mixture into a 9x13-inch pan. Bake for 1 hour, stirring every 20 minutes.
Pour onto wax or parchment paper to cool. Break into pieces.