Boeing 737 Max jets in a deal valued at about $7.5 billion, a person familiar with the talks told Reuters on Sunday.
Wording of the impending deal, which confirms a Reuters report from last week, comes as U.S. President Joe Biden visits Hanoi to boost ties with the former foe.
Boeing and Vietnam Airlines did not immediately respond to requests for comment.
The deal would be worth up to $7 billion at list prices, and usually companies that buy in bulk get discounts, but the person familiar with the talks did not comment on this.
Boeing has a deal with Vietnam Airlines' rival VietJet for the sale of 200 of its 737 MAX planes.
Vietnam was the world's fifth fastest-growing aviation market in 2022 when it lifted COVID-19 travel restrictions, according to the International Air Transport Association (IATA), which predicts the country will serve 150 million air transport passengers by 2035.
Boeing 737 Max jets were grounded worldwide for months after two fatal crashes in Indonesia and Ethiopia in 2018 and 2019.
The planes returned to service from the end of 2020.
Vietnam Airlines recorded a net loss of 1.3 trillion dong ($53.96 million) in the second quarter of this year, its 14th quarter in the red, its financial statements showed. The carrier has said the losses were due to rising fuel prices and other financial risk factors.
Boeing has also been in talks with Vietnamese authorities, together with other U.S.