Heat oven to 350°F. Grease a 9x13-inch baking pan.
Whisk cake mix, eggs, water, oil, and vanilla until smooth, about 2 minutes.
Pour batter into the pan and smooth the top.
Bake until golden and a toothpick tests clean, 26–30 minutes.
Cool the cake 10 minutes. Poke deep holes all over with a wooden spoon handle.
Drizzle condensed milk over the warm cake, letting it seep into holes (optional).
Whisk pudding mix with cold milk for 2 minutes until slightly thickened.
Pour pudding over the cake, nudging it into holes. Chill 30 minutes to set.
Layer sliced bananas evenly over the pudding.
Spread whipped topping over bananas. Sprinkle with crushed wafers.
Chill at least 2 hours before slicing for neat pieces.