To find the Randgate Security Key in Dead Island 2, travel to the spot in Ocean Avenue shown on the below map.
There you’ll find Randgate Staff. Once you take them down, loot Randgate Security Key. Now that you have the key, head east to the Randgate building.
Inside the room next to the entrance you’ll find the Randgate Secure Case underneath the desk. Unlock it to receive the Vengeful blueprint.