In a 2 litre saucepan, combine corn syrup and brown sugar,. Cook and stir till mixture comes to a full boil. Stir in peanut butter. Remove from heat. Stir in cereal, chocolate pieces and vanilla. Stir in cereal, chocolate pieces and vanilla. Press into an ungreased 9x9x2 inch pan. Chill 1 hour. Cut into bars.