Bring sugar, margarine and syrup to a boil in a quart measuring cup. Boil 2 minutes on High in microwave oven. Remove from oven. Stir in soda and vanilla. Already have popped corn in a large brown grocery bag. Pour syrup over popped corn. Fold over top of bag and place in microwave. Microwave for a total of 4-1/2 minutes on High. Remove bag from oven every 45 to 60 seconds. Shake well and turn bag over. Spread caramel corn on waxed paper to cool. Store in tightly covered container.