When you get to the marker to get ready for the smuggler, you’ll find a gyrocopter waiting for you, so take that and follow the guy and his entourage. Once you see him going off-road, that’ll be the location of the vault, so land and get ready for some action. Of course, if you have the grenade launcher sidearm, then you can rain some explosive goodness on the enemies below.
Once the party is over, loot the smuggler for his key and go into the vault. Snatch the diamonds from the table and exit the vault to find a welcoming party for you. One of those enemies will be a Heavy, so take your time and use the inside of the building for cover if needed. Bring the diamonds to the drop-off to finish the mission.