Preheat oven to 350 degrees F. In a medium bowl, stir together flour and brown sugar. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Press onto the bottom of an ungreased 13x9x2-inch baking pan. Bake for 15 minutes. Sprinkle pecans and coconut over hot crust.
Meanwhile, in a small bowl, combine sweetened condensed milk and vanilla. Carefully pour over pecans and coconut, spreading evenly.
Bake for 25 to 30 minutes more or until filling is set. Cool in pan on a wire rack for 10 minutes.
Meanwhile, in a small saucepan, combine caramels and milk. Cook and stir over medium-low heat just until caramels are melted. Drizzle caramel mixture over filling. Sprinkle with chocolate pieces. Cool completely. Cut into bars. Makes 24 bars.