Your automatic weapons are not that accurate to keep severing brainstems, your just chewing up ammo boys. And about the bank vault.
As the group enters the vault and attempts to lock the door they run into a problem. The vault used to be on a hydraulic pump, but it would seem that early in the days of the infections the vault door was blown open by robbers destroying the hydraulic system, the sanctity of the door, and twisting the portal in which it sat. The squad effectively worked themselves into a corner, and just trapped themselves from being able to use the "ooh shit" grenades. Zombies began to swarm in and effectively filled in the banks lobby.