Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan.
In a large bowl, sift together the flour, sugar, baking soda and cocoa. Measure and combine water, oil, vinegar and vanilla. Pour liquid ingredients into flour mixture and beat until smooth. Pour into prepared pan.
Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.