Dao Quang Thuc had called on Facebook for environmental protections and his country's territorial rights in the South China Sea.
Duong seeks appeal of “disturbing public order” ruling by court in Bac Ninh province.
Nguyen Van Tuc, a member of the Brotherhood for Democracy, had criticized the government in writings online.
Nguyen Trung Truc, a member of the online Brotherhood for Democracy advocacy group, had been convicted of working to overthrow the government.
Vo Tan Minh, a drug trafficking suspect, died in Sept. 2017 after being beaten and kicked in a cell by detention center guards.
The move is seen as a further attack on freedom of expression.
Relatives seeking answers are sent first to one place then another by police, family members say.
The clash comes two days after residents block a highway to protest operations at a polluting waste-processing plant.