Preheat oven to 350ºF. Drain pineapple and reserve 2 tablespoons of juice; set aside. Melt 2 tablespoons of margarine and pour into the bottom of a 9-inch round cake pan. Top margarine with brown sugar and, using a fork, swirl together to make an even layer. Place 1 pineapple ring in center of pan and arrange 6 more rings around it. Place cherry halves, cut side up, in center of each ring and in spaces between rings; set aside.
In a medium bowl, combine flour, baking powder, baking soda and salt; set aside. In a small bowl, combine milk, 2 tablespoons of reserved pineapple juice and vanilla. In a large mixing bowl, beat together remaining 4 tablespoons of margarine and granulated sugar. Add eggs, one at a time, mixing well after each addition. Add flour mixture and milk mixture in small batches, alternating each and beginning and ending with flour mixture; mix until blended. Pour batter over pineapple in pan.
Bake until a wooden toothpick inserted near the center of cake comes out clean, about 30 to 35 minutes. Cool 2 minutes in pan, invert pan onto a serving plate and slice into 8 pieces. Serve warm or at room temperature.