Place above ingredients in a BIG bowl.
Bring to boil butter, brown sugar, syrup and baking soda in a BIG pot. Boil 3-5 minutes stirring constantly. Remove from heat and add vanilla. Pour over popcorn/cereal mixture. Stir to coat.
Divide popcorn/cereal mixture on 4 cookie sheets and bake @ 275 degrees for 15 minutes, stir and repeat again 3 times for a total of 1 hour. If 4 cookie sheets don’t fit in your oven ½ recipe.