Front-rower Vunipola won the last of his 79 caps against Ireland in March 2023 and has toured with the British and Irish Lions on three occasions.
The 33-year-old follows his younger brother Billy Vunipola at Montpellier and ex-captain Owen Farrell at Racing 92 in leaving Saracens for France as English rugby continues to suffer financially.
"A real legend of English rugby," Vannes said in a statement.
"By committing to Vannes, Mako Vunipola will have his first experience of playing abroad and will discover the Top 14," they added.
Breton outfit Vannes have also signed Fiji winger Filipo Nakosi and Chile centre Inaki Ayarza for their maiden season in the top-flight and have been linked with former New Zealand midfielder Francis Saili, currently at Racing 92.
The Vunipolas and Farrell will join other former England Test players such as lock Dave Ribbans at Toulon, back-rower Jack Willis at Toulouse, centre Joe Marchant at Stade Francais and wingers Jack Nowell and Henry Arundell at La Rochelle and Racing 92 respectively in the Top 14.
Prop Kyle Sinckler, back-rower Lewis Ludlam and centre Manu Tuilagi are also heading to the league.
In addition to the arrivals from the across the English Channel in the first division, former international back-rower Courtney Lawes and ex-England winger Jonny May have joined clubs in the second-tier ProD2.
Lawes has agreed a move to 1997 Champions Cup winners Brive while May has signed for Soyaux-Angouleme.
