The violence broke out a day after arson and looting in the south of the country.
A doctor at a hospital in central Ha Tinh province said five Vietnamese workers and 16 other people described as Chinese were killed on Wednesday night in rioting, one of the worst breakdowns in Sino-Vietnamese relations since the neighbours fought a brief border war in 1979.
The doctor at Ha Tinh General Hospital said: "There were about 100 people sent to the hospital last night. Many were Chinese. More are being sent to the hospital this morning."
Local media, however, said only person was killed, while China's state news agency Xinhua reported that at least two Chinese nationals had died and more than 100 had been hospitalised.
Vietnamese Prime Minister Nguyen Tan Dung called on police and state and local authorities to restore order and ensure the safety of people and property in the affected areas.
He said: "Appropriate measures should be taken immediately to help businesses stabilise quickly and return to normal production activities."
The Planning and Investment Ministry blamed the clashes on "extremists" and warned that they could seriously affect investment in Vietnam.
Formosa Plastics Group, Taiwan's biggest investor in Vietnam, said its upcoming steel plant in Ha Tinh was set on fire after fighting between its Vietnamese and Chinese workers.
One Chinese worker was killed and 90 others injured, it said in a statement in Taipei.
The plant is expected to be Southeast Asia's largest steel-making facility when it is completed in 2017. No details of fire damage or financial losses were immediately available, the company said.
The Ha Tinh industrial park, estimated to cost more than £16 billion, is more than half complete.
When finished in 2020, it will have a port, a 2100 megawatt power plant and six furnaces.