Heat margarine, brown sugar and corn syrup. Bring to boil, stirring often. Boil for 5 mins. Very slowly as not to burn - do not stir. Add baking soda and vanilla, then blend and pour over popcorn. Mix and bake in a buttered roasting pan. Bake mixture at 250 degrees for 1 hour. Stir every 15 mins. With spatuala. Stir occasionally while cooling to prevent from sticking to pan. Store in airtight container.