Callahan for Vanaduke.
The Argent Peacemaker is just as useful as the Sentenza against Vanaduke, since he is weak against piercing and resistant to everything else. Since the Callahan is pure piercing it will get the full bonus against him, where the argent gets only half bonus. Though the Argent may be better if you will be using only one gun, since Citadel is full of Undead (which are weak against elemental)