Three US citizens and four Vietnamese have been sentenced to 15 months jail for terrorism, a Hanoi court has ruled.
The group tried to air opposition messages, the court heard
Taking into account time already served the seven could be freed in a month, when the US citizens face expulsion.
The seven were accused of trying to set up illegal radio transmitters to make anti-communist broadcasts.
Correspondents says the case threatened to complicate ties with the US ahead of President George W Bush's visit next week and a Congressional vote on trade.
All of the seven defendants are of Vietnamese origin, but three - Nguyen Thuong Cuc, also known as Cuc Foshee, Huynh Bich Lien and Le Van Binh - also have US citizenship.
Local press reports have linked them to a California-based anti-communist organisation called the Government of Free Vietnam.
They are alleged to have brought transmitters and other equipment into Vietnam from neighbouring Cambodia.
They were hoping to take over local radio stations and broadcast anti-government radio messages, according to the BBC's correspondent in Hanoi, Bill Hayton.
The charges carried punishments ranging from up to 12 years jail to execution, but prosecutors said their more lenient sentences reflected the fact the defendants had repented and had no previous criminal records.
The case was heard exactly a week before President Bush arrives in Vietnam to attend the annual Asia-Pacific summit (Apec).
It also threatened to complicate scheduled votes in the US Congress intended to permanently normalise trade relations between America and Vietnam, our correspondent says.
Senator Mel Martinez from Florida, the home state of one of the accused, has threatened to block the bill because of the case.
That would be an embarrassment to both governments, which have heralded the bill as symbolising their new partnership.
The Vietnamese government is currently trying to extradite a man it calls the leader of the plot, Nguyen Huu Chanh, from South Korea.
An earlier attempt failed. Mr Chanh was one of the founders of the Government of Free Vietnam.