As far as I know all of the vertex_index functions in the module currently don't use the graph, and just use the underlying vertex_index(u) method. If there is a plan to implement methods that give indices for simpler lists such as GenericAdjacencyLists this makes sense, but in the meantime it is just a little confusing. When drafting my code I started passing the graph itself to methods where it wasn't necessary in the first place.