Preheat oven to 350'F.
Add chopped rhubarb, 1 cup brown sugar, 3 tablespoons of flour, vanilla and salt to a bowl and stir to combine. Set aside.
Whisk together oats, remaining flour and brown sugar, cinnamon and baking powder. Pour melted margarine over and stir until well combined. It will be a little crumbly and that is fine.
Pour rhubarb mixture into a 9"x13" casserole dish or high sided baking pan. Sprinkle the crumble mix evenly all over and place in the oven to bake.Bake at 350'F for 40 minutes, until top is golden brown and rhubarb is tender, juicy and slightly thickened.
Serve warm with vegan vanilla ice cream or whipped cream.
