Whisk together first 4 ingredients in a large saucepan over medium-low heat. Cook, whisking constantly, 20 minutes or until thickened. Remove from heat; stir in butter and vanilla until butter melts. Let stand 10 minutes.
Arrange half of vanilla wafers evenly in a 13- x 9-inch baking dish; top with half of banana slices and half of blueberries. Spoon half of pudding mixture evenly over fruit. Repeat layers. Cover and chill 4 hours.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add powdered sugar, beating until soft peaks form. Spread evenly over chilled pudding. Serve immediately.