Ten Vietnamese accomplices also received jail terms of between 5 and 11 years on a charge of helping in a plot to overthrow the government.
The two day trial in Ho Chi Minh City was widely covered in state media, as the government attempts to highlight what it calls a terrorist threat from anti-communist organisations based in the United States.
Nguyen James Han, 49, and Angel Phan, 62, were accused of being members of the California based Provisional National Government of Vietnam, a shadowy group with no official recognition that claims descent from the South Vietnamese government defeated in 1975.
Prosecutors said they had plotted to bomb the international airport in the southern metropolis and set up bases to carry out other terrorist acts.
There has been no reaction from the group or its leader, Dao Minh Quan, who claims to be the prime minister of a government in exile.
The US embassy in Hanoi has also made little comment about the case, saying merely that it will provide consular assistance to Han.
The Vietnamese government says the pair were infiltrated into the country in 2015 with the aim of rallying resistance to the communist authorities.
It says that they had distributed leaflets and attempted to broadcast information on radio stations. It also said they took part in protests against the Taiwanese steel company, Formosa, which caused an environmental disaster on the central coastline in 2016.
Analysts say the authorities are keen to play up the activities of US based opposition groups in an attempt to damage the image of homegrown civil society campaigners.
The government frequently accuses its domestic critics of being beholden to foreign based organisations.
The exact nature of the group’s aims and activities remains unclear. However, the sentence of 14 years for alleged terrorism looks relatively lenient given the 20 year term handed down to a domestic government critic earlier this month.
The government may not take the group’s activities quite as seriously as it is making out.
The court was also told that the alleged plotters suffered from limited education and lack of legal knowledge and had succumbed to blind faith in the group’s promises – a plea for mitigation that is seldom heard in the trials of homegrown bloggers, civil society activists and environmental campaigners.