In Assassin's Creed IV: Black Flag, Charles Vane is portrayed as a historical figure and a character in the game. While he is depicted as a ruthless and unpredictable pirate, there is no explicit indication or storyline suggesting that he suffers from psychosis or gradual onset psychosis in the game.
It's important to remember that Assassin's Creed IV: Black Flag is a work of fiction, and the portrayal of historical figures is often dramatized and embellished for storytelling purposes. The game may take creative liberties with historical characters to fit the narrative and gameplay.
If you are interested in historical accounts of Charles Vane and his life, it's worth exploring legitimate historical sources or biographies to gain a more accurate understanding of the real person behind the character in the game. However, there is no evidence to suggest that Charles Vane suffered from psychosis, as portrayed in Assassin's Creed IV: Black Flag.