Pre-heat oven to 250ºF.
In a large bowl, combine cereal and pecans.
In a medium 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 9 × 13-inch pan.
Bake for 1 hour, stirring every 20 minutes.
Remove from oven; pour onto wax or parchment paper to cool.
When completely cooled, break into pieces.