This is the final mission for Longinus and pretty much plays out the same as previous ones. Your first goal is to reach the vantage point, where you’ll spot the smuggler passing by in his vehicle, along with his escort. When you get to the vault, kill all of the enemies outside of it, including the smuggler (one’s a Heavy Gunner).
Loot his body to find the key that will let you inside of the vault, where you’ll find the diamonds on the table, as in previous missions. Note that the vault entrance is underneath the porch. As usual, as you exit the vault, you’ll find some enemies waiting for you, with the same crowd (with a Heavy and sniper). Bring the diamonds to the drop-off to finish the mission.