Place whipped topping mix, 1/2 cup of the milk and vanilla in a large bowl. Beat on high speed until thick and soft peaks form, about 4 minutes. Add remaining milk and devils food pudding mix. Beat on high speed 2 minutes.
In a small bowl, set aside 2 cups of the pudding mixture (for frosting). Fold the finely chopped wafer candy bars into remaining pudding mixture (for filling).
With a serrated knife, slice the marble loaf cake lengthwise into 3 equal slices.
Place the bottom slice on a serving plate. Spread with half of the pudding-candy bar filling. Top with middle cake layer and remaining pudding-candy bar filling. Top with remaining cake slice.
Frost the top and sides of cake with reserved 2 cups frosting. Garnish top of cake with the coarsely chopped candy bar. Refrigerate for at least 2 hours before slicing.