I agree with DC that I can't see Sinatra as Vito. But then again, given Brando's all-time iconic performance, I don't think I could see anyone else in that role.
DS: Thanks for this info, which came as as news to me. But It's not unimanginable. Sinatra fancied himself a tough guy, and he did very well in tough-guy roles. One of his finest, IMO was in a little gem from 1954 called "Suddenly" (with Stirling Hayden). Sinatra was numero-uno.