Even though many Vietnamese have low opinions of the Communist Party, its members generally avoid criticizing it for fear of repercussions that would affect their livelihoods, said Mai Thanh Son, a senior researcher at the state-affiliated Institute of Social Sciences in central Vietnam.

“The expulsion of Tran Duc Anh Son is a thoughtless decision,” he said. “It’s like releasing a tiger into the forest, and it contributes to stripping away the cowardly face of the ruling apparatus that the party represents.”

In January, a cybersecurity law took effect in Vietnam that requires technology companies with users there to set up offices and store data in the country, and disclose user data to the authorities without a court order. Human rights advocates say Vietnam’s new cybersecurity law was meant to let the government better surveil its critics on Facebook, the country’s most popular social media platform.

Facebook declined to comment on the record about Dr. Son’s account.

The Foreign Ministry did not respond to emailed questions about Dr. Son’s expulsion from the party, including whether his criticism of Vietnam’s South China Sea policies had played a role.

Vietnam has clashed repeatedly at sea with China, which claims most of the waterway as its own. Notably, in 2014 a state-owned Chinese oil company towed an oil rig to waters near Danang, provoking a tense maritime standoff and anti-Chinese riots at several Vietnamese industrial parks. The Communist Party likely fears a repeat of such anti-China-fueled Vietnamese nationalism, partly because some critics question why the government does not take a harder line against Beijing.

Chinese officials and scholars seek to justify Beijing’s claim to sovereignty over South China Sea waters that encircle the disputed Paracel and Spratly archipelagos by citing maps and other evidence from the 1940s and ’50s.

But Dr. Son and other Vietnamese historians argue that the Nguyen dynasty, which ruled present-day Vietnam from 1802 to 1945, wielded clear administrative control over the Paracels, decades before imperial or post-revolutionary China showed any interest in them.