Vainikolo (26) has signed a two-year deal with the Connacht club in Ireland. He will join former North Otago loose forward Ray Ofisa and Otago No 8 Ezra Taylor at the club.
Former Highlanders centre Niva Ta'auso also plays for the side.
Connacht is often seen as the fourth province in Irish rugby, behind Munster, Leinster and Ulster. It plays in the Magners League, and sits eighth in the 12-team table.
Vainikolo will not be playing for Otago in the ITM Cup this season.
He said it was a tough decision to leave, as he enjoyed his time in Dunedin, liked the people and enjoyed playing for Otago.
But he wanted to go somewhere else and get a fresh start and had to secure a future for his family. He has a daughter, who turns 6 tomorrow.
Vainikolo had played for the Highlanders for the past three years, but had missed out this season.
Otago Rugby Football Union general manager Richard Reid said Vainikolo has been granted an early release from his contract. It was felt in the best interests of the union and Vainikolo to allow him to take up the Irish contract, Reid said.
Vainikolo was signed by Otago at the end of the 2008 season after he impressed with the Highlanders that year, being drafted from Northland.
But he has never come anywhere near recapturing the ability which had made him hot property in 2008.
He ripped his hamstring while preparing for the Super 14 season in 2009 and never regained any form. His displays were average for Otago in 2009 and again in 2010, his form never scaled any heights.
Vainikolo had been contracted by Otago this year and its wing options will now be provided by the likes of Joe Hill and new signings Hanipale Galo and Buxton Popoali'i.
Ryan Shortland, who also played on the wing for Otago last year, has moved to the Newcastle Falcons and Otago captain Alando Soakai announced earlier week he was leaving to play in Japan after the Super 15.
