1Butter bottom and sides of 13x9-inch pan. Remove and discard unpopped kernels from popped popcorn.
2In large bowl, mix popcorn and peanut brittle. In 4-cup microwavable measuring cup, microwave caramels and water uncovered on High 2 to 4 minutes, stirring every minute, until smooth. Stir in vanilla.
3Pour over popcorn mixture; toss until evenly coated. Press in pan with buttered hands. Let stand about 30 minutes or until set. For 24 bars, cut into 6 rows by 4 rows with wet knife.