Italian Vio has built a reputation as a free-kick expert during previous spells at Catania, Fiorentina and AC Milan, while he also worked at Brentford during the 2015/16 season.
"He's a specialist. He's one of the best in the world," Christiansen said after Tuesday night's home defeat to Derby.
"His CV and all the numbers tell good things about him, what he has achieved in many clubs. One of them is Fiorentina, where he did a fantastic job.
"This is something we're looking forward to. He has many interesting ideas. The club told me there was a possibility and of course I want to have the best with me. I'm more than happy with that."
Vio will join assistant coaches Julio Banuelos and Paul Butler, goalkeeping coach Marcos Abad and fitness coach Ivan Torres in Christiansen's backroom team.