Lightly grease loaf pan. Beat together margarine & sugar till fluffy. Add vanilla and eggs. Beat until blended. Add sour cream and milk and mix well. Stir together flour, baking powder, baking soda and salt. Add to margarine mixture, beating well. In separate bowl mix together the 1/3 cup sugar and cinnamon. Put 1/4 of the mixture (2 Tbsp.) in a cup.
Spoon 1/2 batter in loaf pan. Sprinkle with large amount of cinnamon mixture. Spoon remaining batter over top. With knife, cut thru the batter to form swirls. Sprinkle remaining cinnamon over the top of the loaf.
Bake 60 minutes in 350 degrees F oven. Cool on wire rack 10 minutes then remove from pan.