Preheat oven to 350F. Prepare a 9x13-inch baking dish by spraying with non-stick cooking spray; set aside.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.
Make a large well in the center of the mixture and two small wells next to the large well. This is to keep the wet ingredients separate until all mixed together.
In one small well add the vinegar, in another small well add the vanilla, and in the large well add the vegetable oil.
Pour the water into the mixing bowl and immediately stir to combine all of the ingredients together.
Pour the batter into the prepared baking dish and bake at 350F for 30-35 minutes until a toothpick inserted into the center comes out clean.
Cool the cake completely to room temperature then frost with the frosting of your choice.