The order to cancel the Human Rights Watch event about the mainly Christian ethnic Montagnards comes ahead of a scheduled trip to junta-ruled Thailand by Vietnamese Prime Minister Nguyen Tan Dung next month.
The cancellation, which was announced by police minutes before the briefing was due to start, is the third event this month to be shut down by authorities at the Foreign Correspondents' Club of Thailand.
A few dozen uniformed and plainclothes police officers mobilised outside and within the FCCT building Friday morning, the latest crackdown on free speech under the military regime that seized power last year.
Phil Robertson, deputy Asia director for HRW, said Thailand's order to cancel the news conference was "very worrisome".
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The move "raises questions" on whether Vietnam asked Thai authorities to stop the event, he told reporters who had gathered for the news conference at the FCCT in downtown Bangkok.
In a later statement, HRW said: "Thailand is choosing to side with dictatorships in ASEAN while further stepping up repression at home."
"The event is not appropriate in the current situation," it added.
The FCCT said the police order was issued on behalf of the National Council for Peace and Order (NCPO), the official name for Thailand's junta.
The report at the heart of the issue found that "Highland people accused of religious 'evil ways' and politically 'autonomous thoughts' have been subjected to intimidation, arbitrary arrests and mistreatment in custody" in Vietnam.
HRW called on international donors to urge the country to end its "abusive" policies.