This is the third quest you must complete for Opía Apito. To start it, simply talk to Opía again after finishing the "Templar Ships" quest. She will be waiting for you in the western part of the Grand Cayman settlement.
Edward attempts to locate the Templar's right-hand man.
Follow Opía to the house where Vargas lives.
Hide in the pile of leaves you'll find nearby. This will trigger a short cutscene showing Vargas slipping out of the house and starting to run away.
You actually don't even need to run after him - no matter how hard you try, Vargas will manage to reach the docks, jump onto the ship, and sail away.
Board your ship and take the wheel.
Set off in pursuit of Vargas's ship. You can use travel sails to catch up to him.
Once you get close, shoot his sails to slow him down, then deal enough damage to completely disable his ship and initiate a boarding.
The only person you need to defeat is Vargas - you can ignore the rest of his crew. In fact, you don't even have to step foot on his ship - a single well-aimed shot with swivel guns or pistols is enough to trigger a cutscene in which Edward gets the necessary information from Vargas, finishing the quest and unlocking the next one - The Trail of Lucia Márquez.
