The deal was signed during a visit by Vietnamese prime minister Nguyen Xuan Phuc to the US.
It is the formal beginning of a partnership between Mainstream and GE announced in September 2016. It represents Mainstream's first major investment in Asia, following moves in to Latin America and Africa.
The two-phase 800MW Phu Cuong project is set to become Vietnam's largest wind site, costing approximately $2 billion.
The first 150-200MW phase is expected to reach financial close in 2018, the partnership said. GE is set to supply turbines to both phases of the project.
"This is a significant step forward in terms of taking the first phase of this project into construction next year, and in doing so supporting Vietnam in its aim of delivering 1GW of renewable energy by 2020," said Mainstream chief operating officer Andy Kinsella.
Phu Cuong Group deputy general manager Pham Quoc Anh added: "Producing electricity from wind energy is a new market in Vietnam. As one of the pioneers, Phu Cuong Group wants to work with strong partners with capacity to ensure the quality of the project."
According to Windpower Intelligence, the research and data division of Windpower Monthly, to-date Vietnam has installed 183MW of wind capacity.