Preheat oven to 350 degrees F. Spray your muffin pan with cooking or baking spray.
Sprinkle chopped plums with 1 tablespoon of flour in a bowl, and toss to coat. Set aside. In a large bowl, beat the margarine, white sugar, and vanilla extract with an electric mixer until light and fluffy. Beat in the eggs. In another bowl, sift together 1 1/2 cup of flour with salt and baking soda. Stir the dry ingredients into the egg mixture, alternating with yogurt until the mixture forms a smooth batter. Lightly stir in the flour-coated plums. Evenly distribute batter between muffin cups. Sprinkle the top of the batter with brown sugar.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 45 to 50 minutes. Remove from oven, and let cool in the pan 10 to 15 minutes