Preheat oven to 350 degrees F (175 degrees C). Spray a 15-cup fluted tube pan (such as Bundt(R)) with cooking spray.
Pour cajeta into a microwave-safe bowl. Heat in the microwave until smooth, 20 to 30 seconds. Pour into the bottom of the prepared pan.
Combine cake mix, water, 3 eggs, and vegetable oil in a large bowl; beat with an electric mixer on low speed until moistened, about 30 seconds. Increase speed to medium and beat until batter is smooth, about 2 minutes.
Pour cake batter over cajeta in the pan.
Combine condensed milk, evaporated milk, 5 eggs, cream cheese, milk, and vanilla extract in a blender; blend until smooth.
Pour milk mixture slowly over cake batter.
Wrap pan tightly with aluminum foil; place in a large baking dish. Pour enough water into the baking dish to come 2 inches up the sides of the pan.
Bake in the preheated oven until a toothpick inserted into the cake comes out clean, about 2 hours. Remove pan from the baking dish; let cool for 15 minutes. Unwrap the aluminum foil; invert cake onto a large rimmed plate. Let cake cool completely, about 30 minutes. Chill before serving.
