Mainstream will develop three projects in partnership with local developers. This marks the first investment in Asia for the Irish firm.
The 800MW Phu Cuong project will cost $2 billion. The first 150-200MW phase is expected to reach financial close in 2018, Mainstream said. Phu Cuong will be installed in the Soc Trang province in south Vietnam.
The 800MW site will be the first to be backed by the Mainstream and GE Financial Services partnership, formed in September, to fund the installation of 1GW of wind capacity in the Southeast Asian country.
Mainstream has also acquired two projects with a combined 138MW. The 83MW Thai Hoa and 55WM Thai Phong projects will be built in Binh Thuan, also in the south of the country.
The Irish developer will partner the projects' original owner, Pacific Corporation, and invest $200 million in the two sites, with financial close also expected in 2018.
"In terms of energy needs and requirements, Vietnam is one of the fastest growing markets in south-east Asia. Investing in Vietnam is very much aligned with our aim of developing large-scale renewable energy projects in high-growth economies with significant demand for new-build power generation," said Mainstream chief operating officer Andy Kinsella.
The Vietnamese government has set out targets of generating 800MW by 2020 and 6GW by 2030, according to Mainstream.