I see what you are saying, I suppose as you said, Vito killed the 'Black Hand' and started the whole thing off by helping people like himself with favours, but Michael and his brothers had grown up seeing Vito as the successful, powerful Don, probably not realising where that power originated from.
TPM

"My wife was weeping before she fell asleep, outside my window I saw my Caporegimes coming to the house and it is midnight. So, Consigliori of mine, I think you should tell your Don what everyone knows."