That's the downside of morphs; a morph will always translate each vertice involved in a straight line from where it was to where it wants to be now. So drawing a bow works well in a morph, but spinning the cylinder on a revolver does not. Since few garments are modelled with an actual split in the front, you are going to need to change the base mesh before creating the zipper morphs. And that means you will lose any existing morphs (body shaping, breast size, et al). I have a feeling, depending on what kind of garment you are doing and what the intended effect is, you are going to end up with a number of different morphs to follow a whole zipper down.