Drain pineapple, reserving 1 tablespoon juice. Measure 1/2 cup pineapple; reserve remaining pineapple for another use. Combine 1/2 cup pineapple, banana, and sugar in a small saucepan.
Combine cornstarch and reserved 1 tablespoon pineapple juice; add to fruit mixture, stirring well. Cook, stirring constantly, over medium heat until mixture comes to a boil and thickens. Remove from heat, and stir in vanilla.
Cut cake horizontally into 4 equal slices. Place bottom slice on a serving platter. Spread 1/3 of pineapple mixture over cake slice. Top with another cake slice. Repeat procedure 2 times with remaining pineapple mixture and cake slices, ending with cake slice.
Spread whipped topping over top and sides of cake. Gently press coconut on top and sides of cake. Cover and freeze at least 4 hours.