Welcome, SNJ!
The "you've found your Luca Brasi" scene that you're referring to appears in the novel. Hagen, not Vito, made that statement to Michael. Much as I don't like to disagree with my friend Goombah, I don't recall it in the Saga.

The question of why Vito didn't prevent Connie from marrying Carlo has come up many times on this board. Carlo certainly was a dirtbag From Da Woid Go, and Vito obviously saw through him. For all we know, Vito did remonstrate with Connie, but perhaps out of sentiment for his youngest child, and maybe to avoid domestic discord, he stopped short of forbidding her to marry him. Or, he may have taken the attitude that it was her decision to make--and she'd have to be responsible for the consequences (as in Vito telling Sonny, "Never interfere between a man and his wife"). Michael seems to have adopted a form of that attitude toward Connie. It might seem horrendously cruel and cynical for Michael to have Carlo whacked on the very day that he stood godfather to Connie and Carlo's infant son. But in the novel (and partly in the film) he rationalized it by saying it wasn't his idea to be godfather--it was Connie's idea. And how about murdering Fredo? Well, it wasn't his idea to "forgive" Fredo--it was Connie's request. Or, as Flip Wilson used to say on TV, "the devil made me do it."