Pop kernels in two batches, using 1/2 cup kernels for each batch.
In a medium size glass or ceramic bowl place margarine, sugar, syrup, and salt. Cook 4 minutes on high in microwave. Add baking soda and vanilla and stir to combine.
Pour popcorn into a large brown paper grocery bag. Pour syrup mixture over popcorn while shaking bag. Fold bag closed. Shake well.
Put bag in microwave and cook 1/2 minutes on high. Remove and shake well. Return to microwave and cook 1 minute on high. Remove and shake well. Return to microwave and cook for 30 seconds on high. Remove and shake well.
Put into a large bowl. Let cool and eat.