The 32-year-old former England international's Kingsholm contract was set to expire at the end of the season but he has been allowed to leave immediately to join the French Pro Division 2 outfit.
Vainikolo joined Gloucester from Bradford Bulls in 2007 and made an immediate impact by scoring five tries on his debut against Leeds.
He was quickly promoted to the England squad and made his Test bow as a replacement in the defeat to Wales at the start of the 2008 RBS 6 Nations, and went on to win four further caps.
The popular Tonga-born three-quarter, nicknamed 'the Volcano' made 86 appearance for the Cherry and Whites, scoring 36 tries.
Gloucester head coach Bryan Redpath said: "No one can deny the impact that Lesley has had on Gloucester Rugby and on the English game in general.
"He adapted from league to union in no time at all and could always be relied upon to give 100% whether in his preferred position on the wing or in the centre when we asked him to.
"La Rochelle approached us to ask permission to talk to Lesley and explained that they were prepared to offer him a two-and-a-half year contract if we were willing to allow him to leave immediately.
"Lesley has been a fantastic servant to Gloucester Rugby and, frankly, it would have been unfair of us to stand in his way. He travelled there this week for a medical and to discuss terms and it's a great opportunity for him.
"He goes with our best wishes and our thanks. He'll always be remembered here at Kingsholm and will always be welcome."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.