Bake the vanilla cake according to package instructions and let it cool completely.

In a mixing bowl, combine the heavy whipping cream, powdered sugar, and vanilla extract. Whisk until soft peaks form.

Cut the cooled cake into cubes. In a trifle dish, layer the cake cubes, mixed berries, and whipped cream. Repeat the layers until the dish is full.

Refrigerate the trifle for at least 1 hour before serving to allow the flavors to meld.