Preheat oven to 350~. Grease and flour two 9" round pans. Mix cake mix, 3/4 cup root beer, eggs, oil and pudding mix in a large bowl. Bake as directed on package. Chill cake for ease in splitting layers. Split each layer into 2 thin layers.

Filling & Topping: Combine Dream Whip, ice cold root beer and vanilla in a chilled bowl. Beat as directed on package. Spread 1 cup of filling between layers of cake with remaining on top. Serve immediately or store in refrigetator until ready to serve.