Place a rack in the center of the oven and preheat the oven to 350 degrees.
Grease and flour a 9x13" cake pan.
In a large mixing bowl, place the cake mix, sour cream, water, oil, eggs, and vanilla. Mix with an electric mixer on low for 1 minute.
Stop the mixer and scrape down the sides of the bowl. Increasing the mixer speed to medium, mix for 2 more minutes, until the batter is well blended.
Pour the batter into the prepared pan, smoothing the top with a spatula.
Bake the cake for 38 minutes. It's done when it springs back when lightly pressed with your finger. It might need up to 4 more minutes, depending on your oven.
Remove the pan from the oven and cool, in the pan, on a wire rack.