Preheat oven to 350°F (175°C) and grease a 9x13-inch baking dish.
Prepare the cake batter according to the package instructions using water, oil, and eggs.
Pour batter into the baking dish and bake as directed on the box. Let the cake cool completely.
Once cooled, use the handle of a wooden spoon to poke holes evenly across the cake.
In a bowl, dissolve the strawberry gelatin in boiling water, then stir in cold water.
Pour the gelatin mixture over the cake, ensuring it seeps into the holes. Refrigerate for 1 hour.
In a mixing bowl, beat cream cheese until smooth. Add powdered sugar and vanilla, mixing until combined.
Fold in whipped topping and spread evenly over the chilled cake.
Top with fresh sliced strawberries before serving.