Vanitas in his normal form. This form doesn't appear in Kingdom Hearts: For True Story, although it is used all the time by him in Heroes From An Old World due to plot reasons for later chapters.
Vanitas is one of the primary antagonists in the PSP game Kingdom Hearts: Birth By Sleep. Vanitas is Master Xehanort's sole apprentice, and is the embodiment of Ventus's dark side. Vanitas is the first Unversed, therefore making him the creator of all existing Unversed. This also means if Vanitas is killed, all Unversed are instantly destroyed as well. Vanitas is a very powerful foe, and he, unlike Ventus, is proud, cunning, and sadistic in all manners.
In the normal continuity of the Kingdom Hearts series, Vanitas was slain by both Ventus and Aqua, thanks to Ventus defeating Vanitas in their mental struggle and Aqua keeping Vanitas at bay while Vanitas possessed Ventus.
Fanfics
Kingdom Hearts: For True Story
Vanitas's role is a short but important one in KH:FTS. Vanitas first appeared in Chapter 16 of KH:FTS, near the end. Vanitas was reborn due to the fusion of Anti-Sora and Anti-Roxas, forming the reborn Vanitas. After announcing that he was the "true Anti-form" and the "Unversed Lord," Vanitas proclaimed his name and served beneath the transformed Namine while fighting off literally everyone that stood before him; it helped that he managed to summon masses of Unversed to help do his dirty work while he dealt with Roxas. In Chapter 17, Vanitas shows much strength ass he single-handedly puts Roxas on the ropes by using his most powerful attacks (which resemble the powerful Vanitas Sentiment). While Vanitas torments Roxas by calling him "Ventus," his Unversed minions manage to kill off Xaldin, Maleficent's gang, and Marluxia. Vanitas then knocked Roxas down and explained to him that, despite his original destruction, he continued to exist thanks to Ventus's heart merging with Sora's. However, due to Roxas's birth, his memories were split into the darkness of both Roxas and Sora; this means half of Vanitas's memories were engraved withing Anti-Sora, while the other half were inside Anti-Roxas. Vanitas then tries to kill Roxas to get to Ventus's heart so the X-Blade can be recreated, but Roxas realized a flaw in Vanitas's plan. If Anti-Sora and Anti-Roxas made up Vanitas--Ventus's dark half--then Roxas and Sora combined makes Ventus. Roxas tried out this theory and was ultimately turned into Ventus-Roxas; aka Ventus's body with Roxas's mind. Ventus-Roxas then dragged Vanitas to the Awakening of the combined heroes, which had a glassy floor containing Sora, Kairi, Riku, Goofy, Donald Duck, Ventus, Vanitas, Aqua, Terra, Master Eraqus, Xion, the entire Organization XIII, and Roxas himself. Ventus-Roxas and Vanitas then brawled for the near-entirety of Chapter 18, while everyone else fought against the powerful Namine. However, soon enough, Vanitas met his end in Chapter 19 thanks to the combined efforts of Ventus-Roxas, King Mickey, Aqua, and Zexion.
Heroes From An Old World
Vanitas makes his first appearance whenever Scaith revives him from his state as the Vanitas Sentiment. Vanitas soon allies himself with Scaith due to the fact he thinks Scaith can give him the opportunity to bring him back the X-Blade. Vanitas is rather downplay for his first couple of scenes, sending out his Flood Unversed to scout the unknown world he was on. Soon enough, he fights some of the protagonists of HFAOW in the glaciers near Dr. Madness's ice base. While Warp, E-1,000 Theta, Gamble, and Furry are off saving Nebula, Vanitas fights Scott and Aero, easily taking care of Scott. This cruelty towards his friend forced Aero to turn into Darkspine Aero, where he soon fought Vanitas head-on. However, Vanitas--bored of the fight--summoned a Trinity Armor to take care of Darkspine Aero while he took Scott to the Realm of Darkness. Once inside the Realm of Darkness, Vanitas attempted to turn Scott into an Unversed, but Aqua interfered quickly and gave Scott time to recover. Vanitas was soon beaten by the combined efforts of Aqua and Scott, but he resurfaced quickly using some Darkside Heartless. However, in a move similar to Aqua's ending in Kingdom Hearts: Birth By Sleep, Terra's and Ventus's Keyblades shone through the Realm of Darkness and killed all 6 of the menacing Heartless, which also knocked Vanitas to the group and made him subject to a final attack by Aqua and Scott. Vanitas fled the scene, fixing his damaged helmet and soon reappearing in the glaciers. Vanitas looked down and saw Warp and Ultranos in the midst of a fight, and he intervened quickly and sent Warp, Ultranos, Nebula, Gamble, and Furry into an underground ice cavern with one sheer impulse to the ground. Vanitas appeared to the five and disabled Ultranos easily, before battling the remaining four.
Vanitas without his helmet on. He bears a striking resemblance to Sora, and takes this form throughout his entire appearance in Kingdom Hearts: For True Story. This form was also seen briefly in Heroes From An Old World, up until he challenged Scott and Aero in the glaciers.