Some Vaesens con only be woundet with holy weapons, but I dont find any rules for holy weapons.
I think they are very rar and uncommen or is a holy weapon simply a weapon who was blessed by a priest (but I think than it will read "blessed weapons").
There is a talent "holy symbol" but thats didn´t refer to a weapon.
At the moment my soulution is a weapon can have the "Holy" characteristic but count´s otherwise as a normal weapon. The background makes the weapon holy nothing else and they can´t be bought. The PC must earn them during other quests.