We all know that Vibhishana was a Brahmin-Daitya/Brahmin-Rakshasa and was an ardent follower of Lord Rama. But I think he didn't kill even a single person in the Rama-Ravana war. So, in my view, he shouldn't be called an asura as he did not have any asura-like qualities.
But does Ramayana talk about Vibhishana being a strong warrior or if he wielded any weapons?
Yes, Vibhīṣaṇa also killed some rākṣasas during the battle between Lakṣmaṇa and Indrajit. Bow & arrow seems to have been his chief weapon.
Seeing Lakshmana and Indrajit, fighting with commitment to gain victory one over the other, like elephants in rut, that strong and valiant Vibhishana, wielding his excellent bow, stood there at the battle-front, with an intent to behold their combat.
Standing there thus, Vibhishana stretched his great bow and released excellent sharp pointed arrows on those demons. Falling in exact spot, those arrows, hitting like fire, tore the demons into pieces, as thunderbolts would cleave large mountains. Even the foremost of demons, the attendants of Vibhishana too, rent asunder those valiant demons in battle, with pikes, swords and sharp-edged spears.
Surrounded by those demons, Vibhishana shone like an elephant in the midst of proud young tuskers. Vibhishana, who knew the timing, spoke to the monkeys, who cherished to destroy the demons, the following opportune words duly inspiring them. "O chiefs of monkeys! Here stands before you, one [Indrajit] who is the last support of Ravana. He is all that is left in his force. Why do you stand idle? At the end of this war, if this sinful demon is killed, the remaining army is as good as killed, except Ravana."
