Dontomasso,Wouldn't Vito have had the same muscle that Michael later used to take out the Dons? All of the shooters were available to him while Mike was enroute.
Not at the point when Michael would have had his "accident." The secret Rocco regime was not yet in place, and the family had suffered heavy losses in the war. Remember to keep the whole thing a secret, it took Michael several years to quietly rebuild the family's muscle, and during that time he had to be humiliated by Moe Green, and had to allow Barzini to chisel on Tessio and Clemenza's territories all without retribution.
Further, I am not sure that with Sonny dead and Vito as sick as he was that an operation as massive as the one Michael pulled off could have been done. All the family had was a sick and aging Vito who had pretty much lost it, Tom "Lets negotiate a deal" Hagen, Tessio (a traitor) and Clemenza.
