New Zealand-based Vui, who can play at lock forward or in the back row, earned his first cap for Samoa against France last month.
"Chris is a big, powerful and adaptable forward," said head coach Carl Hogg.
"He will certainly add some strength to our pack and, at just 23, he is an experienced leader."
Vui captained New Zealand side North Harbour to the Mitre 10 Championship title in October, having played for Auckland Blues in the 2015 Super Rugby campaign.
He previously represented New Zealand Under-20s in the 2013 World Rugby Championship.
