Vuong Van Tha was taken into custody in May, after police laid siege to his home.
His father says authorities never provided a clear reason for the detention.
Hundreds of uniformed officers destroy tents and attack protesters with shock batons and water cannon, sources say.
Foreign ministers of both countries have discussed incident at sea, officials say.
They want Hong Kong-owned Pacific Crystal to permanently shut down the facility and move out of Lai Vu commune in Hai Duong province.
Nguyen Van Oai is sentenced for resisting police officers and leaving his home while on probation.
Group members included pro-government activists and appeared to have official support, priest says.
Nguyen Van Tuc is the sixth member of the Brotherhood For Democracy to be taken into custody in recent weeks.