Lightly grease an 8- or 9-inch (20- or 23-cm) square pan. In a large mixing bowl, mix the flour, cocoa, baking soda, salt, and sugar. Make three holes in the dry ingredients, one large and two small. Add the vanilla to the first small hole, the vinegar to the second and the oil to the large hole. Then pour the water over everything, add the optional vegan chocolate chips and stir to combine.