Hai has posted articles on social media about suspected government corruption and has taken a close interest in the campaign over the environmental disaster that blighted the north-central coast after a toxic leak from the Formosa steel plant in April.
The Communist authorities appear acutely sensitive to criticism of their handling of the leak and the mass poisoning of fish that affected the livelihoods of tens of thousands.
Fishermen and their families in the worst affected provinces have joined a protest campaign, with thousands submitting lawsuits to demand more compensation.
Last month, the human rights blogger, Nguyen Ngoc Nhu Quynh, who wrote under the name of Me Nam, or Mother Mushroom, was arrested and charged with spreading propaganda against the state under article 88 of the penal code.
She also took on a leading role in the campaign for accountability over the Formosa leak.
She has been held in isolation since her arrest on October 10.
The Communist authorities resumed political trials against prominent government critics this year. Independent bloggers say the political mood is deeply oppressive, with their activities and movements closely monitored by the security services.