The letter follows calls for the US president to intervene in Beijing’s growing influence over Hanoi.
Parents say their children's health was ruined in a series of nationwide health scandals, and that they have yet to receive any help from the government.
Chinese-invested businesses in and near the port city pollute freely, ruining the environment, sources say.
Yu Wensheng's case has been handed to local prosecutors, and his wife is now awaiting a trial date.
