Grease a 9"x13" cake pan and set aside. Preheat oven to 350 degrees Fahrenheit.
In the bowl of an electric mixer, combine yellow cake mix, eggs, vanilla, sour cream, melted butter, and water.
Spread batter evenly into prepared 9"x13" pan and bake for approximately 13-15 minutes, or just until a toothpick inserted into the center comes out mostly clean (moist crumbs are good).
While the cake is baking, prepare the peach mixture by stirring together 4 c. sliced peaches and 1/3 c. granulated sugar. Let it sit in the fridge while the cake is baking and cooling so the flavor and juices will come out.
After the cake is cool, spread the peaches onto the cake.
Make whipped cream topping by whipping 2 c. cold heavy cream with 1 tsp. vanilla extract and 1/2 c. powdered sugar in an electric mixer (with whisk attachment on) on high speed until stiff peaks form. Spread whipped cream over peaches and let the cake sit in the fridge until ready to serve.