Took a model that had several named vertice groups already in it. In the vertex modeler, made a selection across all the groups and duplicated it. The duplicates contained the same named groups (which is what I wanted). Took the same model, selected one group and hid it. Made a new selection as before across the remaining groups. ALL groups went away for the entire model; I ended up with no named vertices at all. Was able to reproduce this several times using different selection methods, different orders of hiding selections, etc. The only cure was to select the group I didn't want included in the duplicates and move it, not hide it.