Vito could have avoided being shot if he was younger.....he was slipping, he walked out into the streets alone to buy fruit, and even though they were his streets, he just told Luca Brazi that he didn't trust Sollozzo, so why go out into the streets alone? A younger Don Vito would have never had left his office without a body guard.....Vito's one weakness was his love for his kids.....he loved Fredo too much and trusted him to be a good body guard......Vito never really ever carried a gun, and even if he did, it was his wits that allowed him to be shot, he probably would not have even been able to get the gun out to shoot back.