You have to give Vincent credit for the hit on Joey Zasa. He organized the hit without the help of Michael and carried it out perfectly, even when it was believed to be an impossible task. By doing so he made his bones the same way Vito did when he killed Fanucci and established himself as 'a man of respect'. When it comes down to it this is how your capabilities are measured in the Mafia and Vincent had effectively shown to be a skillful organizer and capable leader.