The media campaigners Reporters Without Borders (RSF) said Vietnamese bloggers living in exile were being censored because of the misuse of a safety feature on Facebook.
RSF said the social network deleted posts and blocked accounts because of alleged violations of community standards.
"Our research shows that the Vietnamese government is apparently abusing digital space to suppress critical voices abroad as well," said RSF managing director Christian Mihr. "Those responsible must stop these attacks and respect the freedom of the press.
Bui Thanh Hieu, a Vietnamese blogger granted political asylum in Germany, was one of those concerned, according to RSF. He writes about social ills in his home country and his Facebook-distributed writings are hugely popular in Vietnam, but he has been repeatedly banned since January.
Hieu was banned from Facebook in October as a "repeat offender" after images from his account were copied and uploaded to other accounts. Those account holders then accused Hieu of copyright infringements.
Vietnamese journalist Trung Khoa Le has lived in Germany since 1993. He runs the online news site Thoibao.de or "Time." He was prevented from publishing a video critical of Vietnam's communist government by a block on his account. Facebook has since admitted there was a "malicious attack" and made some changes.
RSF said there were more than 20 similar cases of writers being silenced.
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Activists in Vietnam also targeted
Blogger and activist Do Cong Duong was given a five-year jail sentence in September on a charge of "abusing democratic freedom." This came just weeks after he was given a four-year sentence for "disturbing public order."
Duong was arrested for posting articles and video reports about land rights and evictions. The most vulnerable sectors of Vietnam's population are concerned, due to rapid urbanization in Vietnam, reported RSF.
Vietnamese activist and blogger Huynh Thuc Vy was sentenced to nearly three years in jail last month. Vy, who is pregnant with her second child, was convicted for smearing white paint on the national flag.
Respect press freedom
Media outlets in Vietnam are strictly censored. People who openly write about workers' protests, land grabs or corruption among high-ranking politicians face persecution and prison.
According to RSF, there are at least 26 imprisoned media workers in Vietnam, where authorities target citizen journalists in particular. On the NGO's World Press Freedom Index, Vietnam ranks 175th out of 180 countries.