This quest begins immediately after completing the Vainglorious Bastards quest. As a result of treachery, Edward and Vane end up marooned on a deserted island. This situation takes a heavy toll on Vane, who slips deeper and deeper into madness.
Marooned and alone somewhere in the West Indies, Edward must survive Charles Vane's increasingly deadly antics.
You start on the beach, stripped of most of your equipment. Head west along the path into the jungle, toward the ruins of an abandoned temple.
When you get close, Vane will start to flee and climb to the top of the ruins. Give chase and climb up after him.
Once you climb the ruins, Vane will start running again, jumping off the viewpoint and fleeing to the north. Do the same - perform a leap of faith and follow the stream downhill.
Do not run along the ground - jump across the branches instead to avoid running into the crocodile swimming in the area.
When you pass through the low gap near the ground, a cutscene will begin - Vane has found pistol and grenades, and unfortunately, he intends to use them against you.
Vane stands on a fallen tree log on the other side of a fairly large clearing and will alternate between throwing grenades at you and shooting. You must run up to him from the left side to get him to stop. He needs a moment to prepare a throw or take a shot - indicated by a corresponding marker so as soon as you see him throwing a grenade in your direction, run behind the stone wall in front of you.
Wait for the sound of the gunshot and quickly move behind the large tree trunk on the left. Then - after another shot - move behind yet another tree trunk.
Run up the slope and then head toward the log Vane is standing on - he will then start fleeing again.
Follow Vane into the jungle. Vane will leave some explosive barrels with burning fuses behind him on the ground. There is no need to rush, so it is best to follow him jumping across tree branches, wooden platforms, beams, etc. That way, you will not only get past the dangerous barrels, but you will also avoid encounters with the jaguars that inhabit the area.
After a short while, you should reach another set of ruins. Vane will climb up them and start shooting and throwing grenades at you once again.
First, you must get close to Vane - who is standing on the upper platform of the ruins, shooting and throwing grenades at you from there. To achieve this, you must first head to the right side of the ruins - which will be the hardest part - and then move to their rear. Although there are plenty of walls here, the lower ones do not offer good cover from Vane's shots. Also, watch out for small powder kegs, as Vane can shoot at them.
The critical spot is the stone stairs at the front right of the ruins. Once you reach them, try to break Vane's line of sight while he is aiming his pistol at you...
...then, as he attempts to throw a grenade, sprint up the stairs and vault over the stone railing on the left to get as close as possible to the base of the temple walls - Vane shouldn't be able to see you here.
Vane should enter search mode. Move along the temple walls to the back and climb to the higher level. You should see a small scaffolding that allows you to climb up to the platform where Vane is.
Climb the scaffolding to the edge of the platform where Vane is moving around, but do not step onto it. Wait patiently for Vane to approach the edge. The knockout prompt should appear for a fraction of a second, so spam the corresponding key / button. This way, you will take him down with a single hit.
Even if a direct fight breaks out between you, Vane is a relatively easy opponent and you should have no trouble defeating him, as the damage he deals is quite low.
After defeating Vane, a cutscene will play - at the end of which Edward safely returns to Great Inagua, completing this quest and unlocking the next one - Imagine My Surprise.