As the number of models added to the VDB grows, the amount of screen real estate given to the Models display increases and pushes the Other Files and Data Roles section past the bottom of the screen. It then becomes impossible to add a data role, edit a data role, or add/remove additional files. At the least a scrollbar should appear on the right of the window to allow you to move to that section of the wizard, but it does not.
The result is that it is impossible to add/administer data roles in any VDB of a reasonable size. I have attached a project with a .vdb that illustrates the behavior.