Prepare the vanilla cake according to the package instructions in a 9×13 inch pan. Allow to cool completely.
In a bowl, mix the sliced strawberries with sugar and set aside for 20 minutes.
In another bowl, beat the heavy whipping cream and vanilla extract until stiff peaks form.
In a separate bowl, whisk together the sweetened condensed milk, vanilla pudding mix, and milk until smooth. Fold in the whipped cream.
Cut the cooled cake into cubes.
In a large punch bowl, layer half the cake cubes, half the cream mixture, and half the strawberries. Repeat the layers.
Top with the thawed whipped topping.
Refrigerate for at least 4 hours, or overnight, before serving.