Easy way to do this (and get the $3M):
Equip a character with any weapon with pestigenocide in the name. Mine was a unique dropped by a Box Dog loot enemy.
There's a high chance that weapon can instantly kill any human enemy if it hits.
Also give your character gear that will give them as high a Speed stat as possible.
My character had 340 Speed and a Unique shotgun named Long-barrel holo pestigenocide equipped.
Go into the vault and take all of the money. Start the fight. Reload and retry until you land a one shot kill.