dt, I agree with you that Vincent's rise was too fast to be credible. It's one of the many plot flaws that diminish GFIII.
No question that Vincent's rise to power happened too quickly.
But was that intentinal on FFC's part? Was it part of the plot?
It was Michael's desperation to become legitimate through Immobiliare that caused him to make some hasty decisions. It was his thirst for legitimacy, his wanting to "get out" of the life that perhaps made him rush Vincent through this mentoring process. Because in the end Vincent blew it at the opera house. The men that he chose, the twins, were NOT as good as they looked. The revenge that Vincent carried out on behalf of the Corleones was well done, but who really carried it out? Neri, Calo, etc. The experienced people. Michael's people.
It was Michael's rush to install Vincent in a position of power that cost him his daughter, his family, his legitimacy and his peace.
I think that FFC intentionally wrote it that way, intentionally wanted to show that Michael, in his hunger for legitimacy, rushed Vincent along into a position of power and wound up paying dearly for it in the end!
