Dark Vlad is lterally Goultard. Goultard has three personalities, each of them has access to a % of his full potential: Goultard (100%), Dark Vlad (50%) and Vald (10%).
Dark Vlad having 50% of Goultard's strength is probably because Craig's team had to beat him, and so do the players in the MMO.
The story is that after Goultard got cursed by Mongrelamus he was imprisoned in a tower. At some point he was freed by a witch because she needed his help to protect the Crimson Dofus from Shushus. She reminds Goultard of her late three wives and they fall in love, but when the fight comes the shushus take her hostage and one of the villagers shoots her dead before Goultard gives them the Dofus. In mad rage he lost reason and became Dark Vlad, and then killed the shushus and the villagers and took the dofus. Craig's team then defeated him and took the Dofus, which hatched into Arty, and Dark Vlad became Vald and lost his memory.
At some point between the manga and the MMO Goultard became Dark Vlad again and the player defeats him.