Ankhat or whatever his name is in the Nagash books isn't a renamed Vashanesh. He's not Neferata's husband. He doesn't receive a ring from Nagash that controls the other vampires or makes him immortal. He never even meets Nagash, fleeing instead to northern lands after the fall of Lahmia and never serving Nagash in the first place. In fact, the only one of Lahmia's original vampires who goes to Nagash is Wosoran, and it is Wosoran and Arkhan who lead Nagash's armies against Nehekhara, not Vashanesh and not Ankhat either. None of the elements of Vashanesh's story are there in Ankhat. So no, the Nagash books don't rename Vashanesh. It's not even like with Ushoran, who is vastly and arbitrarily changed, but still somewhat recognizable. Vashanesh is simply removed wholesale, replaced by an entirely different character.