This quest will unlock after completing the Do Not Go Gently... main quest. To start it, sail to the location marked on the map, located west of Great Inagua. You will find Vane's ship there.
Edward and Charles Vane decide to search for a Royal African Company slave ship to get information about a vessel called the "Princess".
Vane's ship is at anchor, so simply sail up to it and dock alongside.
Board Vane's ship and talk to him. Together, you will agree on a plan for your next actions.
Follow Vane's ship. His vessel will be slightly faster than yours, so you'll lag behind a bit. Sail after him, watching as he attacks nearby ship, and after a short while you should approach a Spanish frigate.
The slave ship will start to flee - give chase, but no matter how hard you try, you won't catch up to it. If you are sailing directly behind it, watch out for the barrels it leaves in its wake. After a moment, you will see the Spanish ship severely damage Vane's vessel.
Simply approach Vane's ship to trigger a short cutscene where Vane and his crew jump onto your ship.
It's time for a battle with the Royal African Company frigate. There is a hostile fort nearby, so prepare for the presence of other enemy ships in the area as well. Do not get too close to the fort to avoid exposing yourself to unnecessary fire.
The fight with the frigate itself is a typical naval battle - much like the ones you have already fought at this stage of the game. You must reduce the frigate's health bar to zero and then board it, followed by bringing its crew's morale to zero.
When the fight comes to an end, a cutscene will begin. Treachery will occur on board your ship, resulting in you and Vane being marooned on the beach of a deserted island. The next quest will start automatically - Marooned.
