In a large bowl, combine flour, sugar, salt baking soda, and cocoa powder.  In a small bowl, combine oil, water, and vingar.  Pour wet ingredients into dry ingredients and stir until well-blended.  Pour batter into sprayed 9x13 pan and bake at 350 for 20 minutes or until toothpick comes out clean.  Remove from oven and cool completely.  To make frosting, combine sugar, margarine, and rice milk in a saucepan and bring to a boil.  Boil for 1 minute, stirring constantly.  Remove pan from heat and stir in the chocolate chips and vanilla.  Beat with an electric mixture for a 2-3 minutes or until it looks like it will set up.  Spread over cooled brownies.