Line a 13-in. x 9-in. pan with foil and grease the foil with butter; set aside. In a large bowl, combine the cereal and nuts; set aside. In a microwave-safe 1-qt. bowl, combine the sugar and corn syrup. Microwave, uncovered, on high for 20-40 seconds. Stir and cook 3-5 minutes longer or until sugar is dissolved.
Stir in peanut butter and vanilla until smooth. Pour sugar mixture over cereal and nuts; stir to coat. Press into the prepared pan. Refrigerate until set.
Using foil, lift out of pan. Discard foil; cut into bars.