Preheat oven to 250 degrees F (120 degrees C). Lightly butter a large roasting pan.
Mix corn and rice cereal, pecans, and almonds together in the prepared roasting pan.
Combine brown sugar, corn syrup, and butter in a saucepan; bring to a boil. Heat to 240 degrees F (115 degrees C) or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface. Remove saucepan from heat and stir vanilla extract into syrup.
Drizzle syrup over cereal mixture and toss to coat.
Bake cereal mixture in the preheated oven for 30 minutes; stir and continue baking for 30 minutes more. Spread cereal mixture in 1 layer onto 2 lightly buttered baking sheets, separating any large chunks with 2 forks; allow to cool.