DANANG, Vietnam — Eight years ago, officials in Danang asked Tran Duc Anh Son to travel the world in search of documents and maps that support Vietnam’s territorial claims in the South China Sea.

He did, and he concluded that Vietnam should challenge China’s activities in waters around some of the sea’s disputed islands, as the Philippines successfully did in a case that ended last year. But his bosses would not be moved.

“They always say to me, ‘Mr. Son, please keep calm,’” he said during an interview at his home in Danang, the coastal city where he is the deputy director of a state-run research institute. “‘Don’t talk badly about China.’”

Vietnam’s top leaders are “slaves” to Beijing, he added bitterly, as torrential rain beat against his windows. “That’s why we have many documents that are kept in the dark.”