Not sure this will solve your issue, but I think the reason its not working is due to Poser not saving dependencies in items which do not belong to that item group or what to say. Which does make sort of sense when you think about it, as you can have several skirts and vests in the scene, so Poser need to know which of them are connected to each other.

The easiest way to get around it, is to "fake" a connection between the skirt and vest. So after you have created your Master morph and place it in the vest for example, you make the skirt a child of the vest. After that you select the vest and save it to the library and Poser will ask you if you want to save the item as an individual item or as a group or something, here you select to save it as a group.

So now when you add it to the scene, it will add both the skirt and vest at the same time and the master morph will be there as well and work on both of them. You still have to go through the normal process of conforming each item to your character.

If you want you should be able to remove the skirt from being a child of the vest and the Master morph will still work, if that for some reason is important.