You have two of the same weapon. Both have damage UVs. One is for fiends, the other is for undead. Which will do more against vanaduke.
I am aware of Vanaduke's weaknesses and resists and is leans towards fiend. And that by lore it suggests he's undead. What family does he register as far as the UV is concerned?
I have searched.. yet can't find the black and white answer. Only hints one way or the other.
As far as I know you can't get a damage bonus against Vanaduke from a UV. You'd need a trinket or armour bonus that gives you damage against everything.