Vaal Vitality Mercenary

My mercenary has vaal vitality but he never casts the ability. I have checked both with myself killing enemies, and him killing enemies, as well as my character being at low life with no life regen and he has never once casted it. I was wondering if this is a bug or if there is some interaction I am not understanding.

I noticed the same issue. I'm not sure if the merc himself needs to get hurt first for him to cast it though.

Mine doesn't cast it either. First of all what is it? Since tere is no sauch skill gem.