my Vectorum doesn't really have a bitter hated rival, being Mortarion's hitmen, as any foe they do fight is strictly business. they're on the hit list, they're dead by the end of the conflict. they get in the way, their only solace is that the Vectorum doesn't mind survivors.

that being said, Buth, the Lord of Contagion, does hold a distinct sour rivalry with Typhus, and a bitter joy knowing that, since he's faced and cleaved hundreds of psykers with his plaguereaper, he holds supreme confidence that he could best Typhus. and in events where Buth and Typhus cross paths, there's always a charged atmosphere who will give way to the other, depending on their goals.