I agree that it was kind of unnecessary to make Vincent Sonny's illegitimate son. Why couldn't they have used one of Sonny's real sons? Or one of Connie's?

Well, maybe Michael would have objected to his sibling's "real" children being involved with the family business, and it was OK for Vincent to get involved because he wasn't a real Corleone. Who knows, but I agree, it wasn't explained well enough.

As for the end of GFII being a good ending to the Godfather saga, I agree. It is implied that Michael is alone and regretful for his actions. But The Godfather Saga begged to be a trilogy, and it allowed fans to go back into that Corleone universe once more, which I am thankful for.

And although there are a few things in GFIII that are recycled from Parts I & II, that's what a Part III does in a trilogy. It brings everything to a close while going through the first 2 movies and bringing back various themes and imagery. Here I think Part III does a good job in that regard.

Sure Part III is clumsy, especially in the beginning, but once it gets moving in the final act, it can't be stopped. It's ambitious and at times, just as good as the earlier movies. And it definitely has the best ending, IMO.