If I search the complete road name:
Via Giuseppe Di Vittorio Vanzago
I got results.
Very often users seach for surname only, so enter:
Via Di Vittorio Vanzago
In that case I got no results.
Is this a search engine bug?
added alternative name as
Via Di Vittorio
There is plenty of documentation about the search engine Nominatim at http://wiki.openstreetmap.org/wiki/Nominatim
Also look at all sub pages there, even an FAQ is there.
Or go to help.openstreetmap.org and search for Nominatim there.
Maybe you are not the first one asking how exact a text has to be entered in the searchbox of Nominatim.