Why not just have it take place...
During the zombie outbreak. The best part of Army of the Dead was the opening title sequence. When the zombies were infecting Las Vegas.
Since it's a prequel, Why not just have a heist movie take place during a zombie outbreak? Trying to execute a heist & escape a city being overtaken by zombies.
Seriously though, the concept is pretty solid & fun. But the execution is anything but that