When a VHM is activated, it adds the mapping 'VIRTUAL_URL_PARTS': (SERVER_URL, BASEPATH1, virtual_url_path) to the request's 'other' dictionary.  If BASEPATH1 is empty, it is omitted from the tuple.  The joined parts are also added under the key 'VIRTUAL_URL'.  Since the parts are evaluated before traversal continues, they will not reflect modifications to the path during traversal or by the addition of a default method such as 'index_html'.