The novel points out that, at the beginning of the Five Families War, the police brought all Mob activity to a standstill because of McCluskey's murder. But they let up after a while because "they still had mortgages to pay off and kids to put through college." The same would apply if Vito had been killed. Some would bolt the Corleones because they didn't trust Sonny's judgment. But they'd still need payoff money, and they'd be fair game for the other Dons.