Blend the condensed milk and the vanilla together in a bowl. Add the powdered sugar a little at a time, blending each time until smooth. Stir in the coconut. Pat the mixture into a greased 9 x 13 inch pan and chill until firm. Place the chocolate chips in a bowl that is microwave-safe and heat on high 1 minute at a time, stirring after each minute, until melted. This is where the recipes vary. You can cut the bars first (1x2 inches) and dip into the chocolate OR pour the chocolate over the top of the uncut bars and cut after they have chilled and the top is firm. I think for a potluck it's easier to cut the bars later, and either keep them in the pan to take to the potluck or put them on a platter once there.