I looked long and hard for a thread that someone made last year, but couldn't. It was essentially asking why Signor Roberto refers to Vito as "Don Vito" but everyone else refers to him as "Don Corleone". I was watching GF Part II last night and that scene reminded me of that question.

I think I'll hazard a guess:

I noticed that not only does Roberto refer to Vito as 'Don Vito' but that Vito in turn refers to Roberto as 'Don Roberto', I suspect this is because they are seen as "equals" in the social status pyramid, as they both carry the honorific of "don".

It's clearly not an official status of any kind, and potentially could be bestowed upon anyone, say, if you're trying to flatter them or sway them to your view. Could it be the type of thing where if someone is "above" you in the social hierarchy, you could call them "Don ___"? Sort of like how you might call your boss or an elderly man "sir"?

I also noticed that Roberto doesn't give the same respect to Vito in their initial meeting and it isn't until after he "asked around" about Vito that he began referring to him as "don vito". It perhaps undercuts my theory somewhat in that if he's afraid, would he project himself as an equal? Or would he want to show that Vito is above him by referring to vito as "Don Corleone"?

Maybe Furio can extrapolate further on how the title "Don" might be used colloquially in Italy, if it still is (I have no idea).