Concern is growing about the health and treatment of the human rights lawyer, Nguyen Bac Truyen, who state media said was arrested with three other pro-democracy campaigners at the end of last month.
His wife says she has received no confirmation from the authorities about his arrest, the charges against him, or where he is being held.
Amnesty International has warned that Mr Truyen could be at risk of torture and other ill treatment, and expressed concern that he is not receiving adequate care for a pre-existing medical condition.
State media reported on July 30 that Mr Truyen was arrested on charges of subversion under Article 79 of the penal code, a charge that can carry the death penalty.
His fellow members of the Brotherhood for Democracy,
Nguyen Trung Ton, Pham Van Troi and Truong Minh Duc were arrested on the same day, part of the government’s widening crackdown on independent bloggers and other critics.
Mr Truyen is a former prisoner of conscience, who has campaigned for freedom of religion in the Mekong Delta region. He has also provided legal assistance to other activists and the victims of government backed land development projects.
He and the others are accused of being in league with Nguyen Van Dai, a lawyer and founder of the Brotherhood for Democracy, who has been held without trial since December 2015.
Mr Truyen had been closely monitored by the security police following his release from jail in 2010, and was subject to sporadic assaults in the street by plain clothes operatives.
Mr Truyen’s wife, Bui Thi Kim Phuong, told foreign media that she had received no reply from police despite repeated attempts to find out where he is being held.
The authorities routinely deny political detainees access to family, lawyers and other visitors until the very eve of their trials, which normally take less than a day and result in lengthy jail terms.
The refusal to confirm Mr Truyen’s arrest, and where he is being held, appears to take the game of psychological warfare against dissidents and their families to a new level.
Analysts say the Communist party leadership appears emboldened by the Trump administration’s lack of interest in human rights and appears set on stamping out a nascent civil society movement in the country.