Symons' departure was confirmed earlier this month, with a move to France mooted.
And it has now been confirmed that Symons, who turns 29 on June 29, will be joining Rugby Club Vannes, who finished eighth in PRO D2 last season.
The powerful back went out on a high at Saints as he was named man of the match in the club's most recent game, a 16-10 Gallagher Premiership victory at Worcester Warriors in March.
After exiting Northampton, Symons said: "I have really enjoyed my two years with Northampton Saints and I just want to thank everyone at the club for welcoming me as they have done.
“The time has come for me to move on, but I’ve really enjoyed working with this extremely talented group of players and coaches, and I want to thank the club’s medical staff too for working so hard to get me through a really challenging injury last year.”
Symons had arrived at Franklin’s Gardens in the summer of 2018 from Gloucester after stints with Leicester Tigers and Worcester Warriors.
A serious knee injury halted his debut season in black, green and gold after just three appearances, but on returning this campaign, Symons notched up a further 14 appearances for Saints, scoring three tries, including a double against Saracens in October.