I think the reason why Vito did not interfere to stop Carlo's beating of Connie- and I think this was mentioned in the novel- was that Connie was married to him. She chose him. Had they not been married, Vito most definately would have interfered. Disgusting, but kind of like a "she's not my problem / property" type attitude.
Actually, Vito was showing his wisdom concerning human behavior. Ask any policeman, and they will tell you how difficult it is to answer domestic violence calls. They can appear on the scene and the couple with be at each others throats with guns and knives, but as soon as an cop touches one of them, they get really defensive. Lets face it most men have their hands full keeping harmony in their own marriages without having to get involved in their childrens.
