To this day I don't understand why the Vincent character was created in the first place, while there already were plenty of potential "legitimate" characters to use.
If you ask me, it was all about cashing on GF franchise not just with the III, but also with the thankfully never realized IVth installment.
Vincent was a failed attempt of fan-service, IMHO. Sonny was a popular character (certainly more so than Connie), so let's give the viewers his son as a post-Michael anchor man.
And let's make him illegitimate to use a bad pun about how Corleone family business can never be legitimate, since it started in crime. Sententious and unrealistic, but whatever.
Oh, and presumably the whole bastard thing was supposed to make Vincent choosing the life of crime more sympathetic. Except that if he was known to Corleones and regularly invited to family parties, then it is clear that he had more options than run-of-the-mill person anyway, so it didn't work at all, IMHO.
The irony here being that going with solid dramatic potential, rather than shallow marketing tricks not only would have made GFIII much better, but GFIV possible and even desirable. Oh, well...
