At the top of the tower in Downtown, there is a vault door you'll want to unlock to get loads of nice loot, but you need to get three keycards to do so.
One of those keycards is grabbed by completing the Arms Deal Public Event. Here's a guide on how to do exactly that.
How to Start Arms Deal Public Event in Warzone
Once you've reached the Nakatomi Plaza, which is the massive tower in Downtown, look for the ramp that goes down into the parking area under the tower.
If you look at the screenshot below, you're looking for the location in the mini-map. That's where on the tower the ramp is situated.
Once you're down there, look for a ramp down to the next level, which is called P2. That's where you start the Arms Deal Public Event.
You're looking for the truck in the screenshot below, which is slightly lit up in the back. Other than that, it looks like any other white truck in the parking area.
Once you've inside the back of the truck, you'll spot a table and some electronics and you can interact with.
Hold square if you're on PlayStation, X on Xbox, and click on PC to interact with it and the Arms Deal Public Event will start.
Then, you'll be faced by two waves of enemies. They're pretty easy to kill, but will try to take you out with grenades and annoying green lasers.
The second wave will include a brute, which is just a normal enemy with more health.
Once you've taken out the final enemy, whether it's the brute or not, you'll be able to pick the keycard up off the floor.
Now, head up to the 31st floor to open up the vault door.