I thought on this some more. I wonder if mob protocol of what was considered "personal" and what was considered "business" might have been at least part of why Vito refused to interfere in Connie's problems.
Maybe Vito thought it would set a bad precedent within his larger Family if he "corrected" Carlo. In the hundreds of loyal buttons that ultimately answer to the Don there must be at least a few dozen who are abusive to their wives. Would the Don have wanted to get involved in their marital affairs?
As long as you're making money for your leaders and following orders I don't think the bosses care too much about how you treat your wife-unless/until it interferes with business.
And while I think Sonny definitely would have been remorseful about the situation, I wonder if Vito wouldn't have told Connie something like "You married this guy I didn't think too highly of. You're grown. You deal with it".
Of course in the film Vito doesn't see the extent of the beatings. Sonny does.
Interesting thoughts. In a deleted scene of the movie, Vito sees Carlo and Connie arguging and when Sonny tries to intervene, he tells him to sit down and says, "You never interfere between a man and a woman." In the novel, he clearly knew about the beatings because he actually tells a pregnant Connie to go back to Carlo and behave so he wouldn't beat her anymore. I'm glad they never put that in the movie because the Vito that Marlo Brando portrayed was warmer than the Vito in the book. I can't imagine Brando's Vito saying something like that.
I'm not sure if Vito did know the full extent of the beatings, but I think he was at least aware that Carlo wasn't treating his daughter with the respect she deserved. But he chose to stay out of it, because like you pointed out, Connie chose to be with Carlo.
