The Frenchman, the GT Le Mans IMSA SportsCar Championship class title winner in 2015 with the German manufacturer, will share Vector's LMP2 ORECA-Gibson 07 with Stephane Richelmi and Ryan Cullen in what will be his 16th start at the French enduro.
Gabriel Aubry, who was part of Vector's WEC P2 line-up last year, was originally listed in the car but was not expected to be part of the team's line-up at Le Mans on 15/16 June.
Aston Martin Formula 1 test and reserve driver Felipe Drugovich races alongside Richelmi and Cullen in the European Le Mans Series but was never part of Vector's plans for the 24 Hours.
Vector team boss Gary Holland said: "We are very pleased to welcome Patrick: he is a proven winner and a great addition to the team.
"His previous experience in both LMP2 and GT will be extremely valuable as we aim for the top step at La Sarthe."
Holland also thanked Porsche for allowing Pilet, 42, to take up the drive.
Pilet added: "I am really looking forward to working with the team at Vector Sport.
"We have known each other a little while and I am very happy to have the opportunity to work together after trying to do so for some time.
"Le Mans is my favourite race and I am massively motivated to compete for the win in the super-competitive LMP2 class."
Le Mans this year will be Pilet's fifth outing at the wheel of a P2 ORECA in the big race.
He drove for the IDEC team in 2000-22 before moving to Graff last year.
Before that he was a stalwart of the Porsche factory GTE Pro squad run by Manthey, racing one of its 911 RSRs every year from 2013.
IMSA SportsCar Championship regular Romain De Angelis will make his Le Mans debut with the Algarve Pro Racing LMP2 team.
