Truong Duy Nhat -- a weekly blogger for the Vietnamese service of Radio Free Asia, which aims to provide news to countries that lack press freedom -- last made contact with editors on January 26, a day after he applied for refugee status with the UN High Commissioner for Refugees in Bangkok, the US-funded network said.
"We are extremely concerned about the safety and wellbeing of Truong Duy Nhat," Radio Free Asia president Libby Liu said.
"We hope to hear from him as soon as possible about his whereabouts and to be assured that he's not in any danger," said Liu, whose organization has alerted the US State Department and lawmakers.
Bui Thanh Hieu, a Vietnamese blogger who lives in Germany, alleged in a Facebook post that the Hanoi government felt threatened enough to abduct Nhat.
Press freedom group Reporters Without Borders called on Thailand to investigate, saying the case sent an "absolutely terrifying" message to a community of Vietnamese bloggers who have based themselves out of Bangkok.
If Thai authorities turn up nothing, "that means that Vietnamese agents no longer bother with international law and violate the sovereignty of a partner country to hunt down critical voices," said Daniel Bastard, an Asia expert for Reporters Without Borders.
Nhat served two years in prison starting in 2014 after blog posts critical of Vietnam's communist leadership. He last blogged for Radio Free Asia on the prospects for change in Vietnam in light of major anti-government demonstrations in Venezuela.
Radio Free Asia quoted an unnamed associate of Nhat as saying he was "arrested" as he went to an ice cream shop inside Thailand's massive Future Park shopping mall.