The powerful Kiwi was the scourge of England in the 2008 World Cup, after which he was named international winger of the year, and switched his allegiance to Tonga after winning 28 caps for New Zealand.
Vatuvei has also played 226 times for the Warriors since making his debut in 2004 but has appeared only once this year.
He is said to have been weighing up his options after battling shoulder, rib, hamstring and knee injuries over the past three years.
Salford, who lost out to Newcastle Knights in a bid to sign another Kiwi international, Shaun Kenny-Dowall, have agreed a deal with Vatuvei that runs to the end of the 2018 season.
Earlier in the week Warriors assistant coach Andrew McFadded suggested Vatuvei may be leaving the club.
"I've been made aware - I know that Manu's been looking at his options for a little while but certainly haven't known any of the details, and don't know that any deal is done," McFadden said.
"But what I do know is that Manu really deserves to finish strong - if that's here or overseas, I'm not sure.
"I hope he gets to get back on the park soon."