The best way to tell if it truly is "missing" is to run a locater search (create a new search for "ModelType Name Equal To VNM"). Right click the VNM model and go to Location. It may be somewhere else in OneClick. If Location entry is grayed out, then most likely the VNM model was "removed" from the Topology. To put it back, just right click the VNM model in the search results window and select Copy. Then go to the Topology view and paste it in.
Hope that helps
Jay