I think Hagen would've approved, however, AFTER the appropriate training that Vincent HAD to undergo, as he is not exactly like his father, like everybody says. Sure he had the obvious temper of his father, but he also has Fredo's sensivity, and Michael's calm manner, and Vito's simple strategy. As Garcia himself has said, Vincent is the combination of all the sons. Like all the sons were combined Vito, Vincent was supposed to be all his uncles and father combined. And being an illegitimate Corleone enhanced his upbringing, as Vito was never a Corleone until named so when coming to America. Vincent was named Corleone by Michael when they came to Sicily.

Its a parallel that I think Coppola would've made more obvious, had he had:

1) Duvall as Hagen back
2) A small rewrite of the original script that had Tom in, in order to make it more detailed like its predecessors.
3) TIME to film the movie, time that was taken away from him for the damn Christmas release. I'd rather see this movie a year later, and complete, then the -still great- incomplete film. Had it been COMPLETE, and as Coppola first imagined it, it would surely be a much more serious enemy in the Academy Awards.

Anyway, that what I think would've been, IMO.