Just to add to what I was saying before:

Vito himself the physical person is not "powerful" the thing that makes him powerful is the connections he has and the men he has working for him.
So say Clemenza and Tessio are good friends and decide they want to take over the family, vito no longer has the power because who does he have working for him? Luca? Sonny's "Crew" if he had one.


Another thing that I read somewhere and I've been looking for but haven't been able to find is this (turnbull will be able to correct it if it is wrong):
The family's TOP enforcer (Luca) was usually very well taken care of, sometimes to the point where he was almost "having a better life" than the Don himself. It was a way of keeping the Enforcer loyal and making sure he never tried anything against the Don, almost as if trying to take over the Family would actually be like a demotion.