The curse of the vampyr, i.e. the branch of bloodthirsty undead the san’layn belong to as well, predates the Scourge. It’s even possible the Scourge did not have a hand in this curse at all, as vampyr is (I think) the real life Scandinavian word for vampire. Scandinavian old timey culture being the bedrock of the vrykul as we know them.
Coupled with the crimson rune-sages we met in Stormheim and a lot of their concept art*, my theory is that the vampyr curse originated with vrykul runemasters who took a bad turn somewhere down the line. Later to be repurposed by the Lich King.
This guys lends some credence to the theory as well.