Heat oven to 350 degrees F. Spray an 8×8 pan with nonstick cooking spray.
In a medium mixing bowl whisk together the flour, sugar, baking soda, and salt.
In a larger mixing bowl whisk together the milk, vegetable oil, vanilla, vinegar, and egg until smooth.
Sprinkle the dry ingredients over the wet ingredients and stir until just combined (stop mixing as soon as no dry spots remain so that you don't overmix the batter).
Pour the cake batter into the prepared pan.
Bake until a tester inserted in the center comes out clean and the cake springs back when pressed on the top, 30 to 33 minutes.
Let the cake cool before slicing and serving. (Note: If adding frosting, be sure that the cake is completely cool before adding frosting.)