So, IF the first shooting had failed (for example, if the gunmen show up after Vito was safely in the car, or Vito had decided to leave the office early), then there simply would've been another attempt.
Assuming the shooters 'botch' the attempt, or perhaps Fredo helps takes some of the shots, Vito is in shape to negotiate. I just kind of think the hit on Vito was a very close call named Fredo. Vito could have escaped w/ less damage.(*edit)
