I agree.
Vincent was at his "intellectual" best when he was scheming to get close to Michael, at the beginning. We see him crashing the party, buttering up Connie ("How's my favorite aunt?") so she can get him an audience withg Michael, and forcing Michael to pay attention to him with his tale of Zasa's disloyalty. Then, when Michael upbraids him in front of Zasa and orders him to "Make your peace with Mr. Joe Zasa," Vincent realizes that his uncle has chosen sides against him. So he improvises bravely by biting Zasa's ear, thus forcing Michael to choose family ties over business. Thereafter, he's mostly a glorified bodyguard with a hot-tempered streak. Not a very good bodyguard, either, considering that his vaunted "best twins" fell down on the job, resulting in Mary's murder and its aftermath.

We should also recognize that Michael was busily manipulating Vincent even as Vincent thought he was manipulating Michael. I'm sure that Michael knew Zasa was a potential threat to him even before his party--and that others might be threats down the road. The ear-biting scene must have cemented in his mind that he needed someone like Vincent--a pazzo with a violent temper--to scare others and walk through minefields for him.
In one of the most insightful passages in the novel, Michael asks his father why he needed "an animal" like Luca Brazi. The Don replies that "there are men in this world who go about demanding to be killed," and "who do a great deal of harm to others." The trick, said the Don, is that since he doesn't fear death, and is thus extraordinarily useful in some circumstances, you must "make yourself the only person in the world that he truly desires not to kill him. Then he is yours." That's Vincent.