Preheat oven to 350°. In food processor, pulse pretzels 15 times or until a few large crumbs remain. In large microwave-safe bowl, heat butter in microwave oven on high 45 seconds or until melted; stir in brown sugar and pretzels. Firmly and evenly press pretzel mixture into bottom and up sides of 9-inch pie plate (it should be tightly packed). Bake crust 10 minutes or until lightly browned; cool completely on wire rack.
In large bowl, with mixer on low speed, beat cream 1 minute; increase speed to medium-high and beat 2 minutes or until soft peaks form. Transfer to small bowl.
In same large bowl, with mixer on medium speed, beat cream cheese, granulated sugar and vanilla extract 2 minutes or until smooth. With rubber spatula, gently fold in whipped cream. Spread filling into crust; refrigerate 1 hour or until set.
In small bowl, stir blueberries, strawberries, raspberries and sugar; let stand 30 minutes, stirring occasionally.