After destroying Sercrent’s soul, Vandalieu had briefly spoken to Eleanora and the spirits of the Subordinate Vampires.

And then Vandalieu had broken and destroyed all of the Subordinate Vampires’ souls.
It was not because they had tried to take Vandalieu’s life. He did that because they had directly ended the life of his father Valen, and because they had tried to kill Tarea.