Stir flour, salt and sugar together in a medium saucepan. Without turning the burner on, add soy milk, one cup at a time, and whisk until no lumps remain.
Place saucepan on medium heat, stirring occasionally until pudding begins to boil. Stir continuously until pudding coats the back of the spoon.
Remove from heat and add margarine and vanilla. Chill at least 20 minutes.
This is quick, simple, cheap, and great filling for cakes and pastries as well as on its own. Try it with sliced bananas.