The Vespa with the attached tube thingy was actually a French weapon (though it used an Italian scooter). The tube is a 75mm recoilless rifle. The idea was to provide anti-tank firepower for paratroops. It actually saw combat, apparently with some success, in Algeria.
Vespa 150 TAP
The gun was intended to be dismounted for firing but could be fired from the scooter in an emergency. (The article explains how a recoilless rifle works). They usually worked in pairs, with the second scooter carrying more ammunition.