In 2009, the winger moved to Otago where he earned a further 15 caps before making his Super Rugby debut for the Highlanders in 2008 against the Reds.
The 26-year-old scored an impressive 13 tries for the Highlanders between 2008 and 2010 and is known for his explosive ball-carrying and solid defensive ability.
Vainikolo stands 182 cm tall and weighs 98 kg and will join Connacht at the start of next season on a two-year contract.
Speaking about his move to Ireland, he said: "I am excited about furthering my career with Connacht. I have watched their progress with interest this season and been impressed with their style of play.
"I'm looking forward to hopefully scoring many tries in the green jersey."
Connacht head coach Eric Elwood added: "We're really looking forward to Fetu'u joining us, he has shown he can score tries from anywhere on the pitch and we are excited to have a player of his skill and expertise joining the team.
"Having a player of his pace and physicality in the backs will add an extra dimension to our game."