SVSG, I disagree with your assessment of the landlord's role. If you remember, he treated Vito disrespectfully at first. He told him to take a hike, and to take Mrs. Columbo and her dog with him. However, Vito politely told him to ask around the neighborhood about him and insisted that he keep the money.

Yes,Signor Roberto was just a guy, but he was a man of status and wealth (landlord) who overestimated his own importance and his imperviousness to men like Vito (similar to Woltz and Geary, actually). By checking Vito's reputation with others in the neighborhood, he immediately realized his mistake (unlike Woltz) and came to Vito, literally hat in hand, trembling like a leaf. And the whole scene, with "the rent-a stays like-a before," just confirms how Vito got his point across without any display of temper, without any threats, simply by the force of his reputation and his nature.