I think a big difference between Michael and Vito was how they dealt with people, or how they persuaded someone. Look at Vito with Woltz, and Michael with Senator Geary, on one hand we have Vito who probably gave the order for someone to kill the horse and put the head in the bed, but on the other hand we have michael, who I have no doubt gave the order for that girl to be killed, and to make geary believe he did it.