Well, he wasn't the MAIN reason, but he definitely was a big one. Yeah, Vincent was mostly a jackass in GFIII, but in my version of GF IV his role would be much less central to the plot (and half his time won't be taken up in icky-poo attempts to seduce his cousin).

He also will be somewhat mellowed out after the Sicily shooting and the influences of his brother Santino Jr. and cousins Mikey and Anthony, and his wife Grace.

And his reckless tyrannical tendencies would be reined in shortly when he does something crazy and gets chewed out by the person who now holds the REAL power in the Corleone Family ... Connie.