Not sure if it's all Vuln Down buffs, or just the one from using Vengeance on Warrior, but I believe it's from just this latest patch and we've got a Japanese description on the English client.
http://i.imgur.com/mAQ4lMh.png
I could have just said we've got it listed as 被ダメージ低下 now in the buff section/battle log but I may as well show the localization team my pretty face and cool axe while I'm at it.