[graph-tool] how to use add_edge_list() and then add isolated vertices?

Hi, I am using add_edge_list and it returns a vector map of vertices it has added, but how do I add in isolated vertices that have no edges (otherwise they would have been put in there by add_edge_list). The vector map of vertices is a mapping between integers -> string vertex names. When I add in isolated vertices, they have string names that somehow have to be incorporated into the vmap that is returned by add_edge_list. Any help appreciated. Thanks!