Vulko’s plan worked, as Arthur did indeed defeat Orm and take back the throne, though he dragged Vulko back to Atlantis and threw him in a prison cell for his crimes.
Since then, while Vulko keeps insisting that he’s looking out for Arthur’s best interests and remains loyal to him, Arthur is still distrustful towards his former ally, seeing him as nothing more than a shrewd political manipulator.
Having grown tired of his life of exile and wanting Arthur to finally become king of Atlantis, Vulko engineered this conflict so that Arthur would have no choice but to overthrow Orm, even going so far as to use the Scepter of the Dead King (which he’d hired Black Manta to steal) to control the monstrous Trench and bring them into this mess.