Mix the apple cider vinegar and non dairy milk to make a buttermilk.
Cream together brown sugar and oil and add the buttermilk mixture.
Add egg replacer and vanilla, then add salt, baking soda and flour.
Mix in rhubarb and walnuts.
Pour into a 9x9 inch greased cake pan.
Blend the topping ingredients and sprinkle over top of the cake.
Bake at 350 for 45 minutes.