It's the longest street that doesn't change of name.
I mean what is extraordinary is not the lenght of the street but the fact the axis of Rue de Vaugirard doesn't change of name like many other axis.
Parallel to Rue Vaugirard in the 15th arrondissement, there is Rue Lecourbe.
Rue Lecourbe becomes Rue de Sevres then becomes Rue du Four becomes Rue de Buci and then becomes Rue Saint André des Arts.
If it keep the same name all over the way, it would be even longer than Rue de Vaugirard.
The former main North South axis of Paris, Rue Saint Jacques becomes Rue du Faubourg Saint Jacques and then Rue de la Tombe Issoire.
On the right bank, Rue Saint-Denis becomes Rue du Faubourg Saint-Denis then becomes Rue Marx Dormoy then becomes Rue de la Chapelle.
When in America, a street can have the same name on tens of miles, in Europe it's pretty usual for a street to change names every few blocks.