The Mercedes-Benz EQ Formula E Team has launched the car that it will compete with in the 2019/20 FE season, its first in the all-electric series.
Mercedes affiliated company HWA raced in the series last year, with Vandoorne and Gary Paffett bringing the team to a ninth-place finish in the team's championship.
Vandoorne has been retained for the sixth season of Formula E, while de Vries, who currently leads the FIA Formula 2 championship, will make his debut in the category.
Mercedes also confirmed that Ian James will lead the team as its team principal this season.
"I'm absolutely delighted to be joining Mercedes," said Vandoorne. "I've already gained experience of the series racing for HWA last year. I am very pleased to have now become an official member of the Mercedes family.
"The arrival of Mercedes on the scene is a great opportunity for all of us. The car looks fantastic. I am looking forward to seeing what we can achieve as a team. The potential is huge, and I simply can't wait to get started."
De Vries added: "It's a very special feeling to be asked to drive for Mercedes-Benz EQ in Formula E, and I am honoured that the team have chosen me as one of their two drivers.
"Formula E is a great platform that has already established itself as one of the biggest and most professional racing series outside Formula 1. It's a great opportunity, for which I'm really grateful to Mercedes.
"I've had an excellent reception from the team, and it's great fun to work with them. But ultimately, it will all depend on the performance on the race track once the season starts. It's all pretty new to me.
"That's why I honestly haven't thought about setting myself any targets for my rookie season. Essentially, Mercedes are aiming to be contenders for race wins and championship trophies. Hopefully, this will happen sooner rather than later. But until then, the team and I expect a steep learning curve before we can become truly competitive."
Bahrain International Circuit - Winter testing