Since Stephen Cannell and Frank Lupo decided the relationship of the arc would be between Vinnie and Sonny, I'd say they had to have known that Sonny would have to be likable guy. (And Cannell cast Ray personally, so he knew just what he could bring to the role.)

I know how frustrating it can be for readers to focus on something other than what I consider the main point of the story. But that's entirely their right!