In a large bowl, combine the caramel corn, pretzels, crackers and 1 cup Reese's pieces. In a large microwave-safe bowl, melt marshmallows and butter; add vanilla and stir until smooth. Pour over pretzel mixture; stir until well coated.
Press into a greased 13x9-in. pan. Sprinkle with remaining Reese's pieces; press lightly. Let stand until set. Cut into bars.