Property Map deep copy

I have an external iterator_property_map to an adjacency_list, using std::vector to store std::string as values. I need to copy this map, but a deep copy so that all the strings are unique to the copy. Is there an easy way to do this? I thought about constructing a new property map from a copy of the underlying vector, but this presents two other questions: (i) will the vertex-to-string mappings of the old map still be valid in the constructed copy? (ii) is there a way to extract the underlying vector from the property map? I couldn't find information on this in the online documentation. Björn