Thanh Tung, 20, of Hanoi skipped class and worked full-time at a factory from 7 p.m. to 7 a.m. at a time when his D4-1 Korean language training visa was about to expire.

Tung is one of many Vietnamese students who have gone to South Korea to study short-term language courses under the special visa program, the most popular among international students due to its simple procedures and high visa issuance rate.

He has temporarily given up the factory job for a week and stayed in his rented room in a small street in Busan since the police are hunting for 161 Vietnamese students who have missed classes at the Incheon National University's Korean Language Institute for more than half a month "without providing any reason." The police have suggested that they left school for illegal employment.

Tung is not on the list of 161 but is nevertheless an overseas student dropping out of school to work illegally.

After completing high school and realizing he had little chance to pass university entrance exams, he had persuaded his family to send him to South Korea for the language program.

Through an overseas study counseling center, he spent three months studying Korean and asked his parents to pay some VND200 million ($8,800) before leaving for South Korea in mid-2017.

Since the beginning he had planned to illegally stay in South Korea and look for work. Tung's family is not poor, but he still wants to work in South Korea for five to seven years to earn some money to start his own business.

When his one-year visa was about to expire, he dropped out of school, changed his residence and applied to work full-time at the factory.

Since dropping out of class, his life is "not very miserable but very lonely." While his classmates are free to hang out, go to school and meet friends, he spends all his time working or sleeping.

"The only way for Tung to relieve his mind is by drinking," Viet Hoang, 20, Tung’s friend and also a language student in South Korea, says.

Many Vietnamese make their way to South Korea under the D4-1 visa program and then drop out of school and go missing. They often end up in restaurants, cafes or factories.

Due to the high incomes they can earn, many people choose to overstay their visa. Tung for instance earns $2,500-4,000 a month, 10 times the average monthly income of workers in Vietnam. With his high school certificate, Tung cannot dream of earning so much in Vietnam where the salary of new college graduates is around $250.

Thuy Trang, 22, a native of Phu Tho, around 100 kilometers to the northwest of Hanoi, went to South Korea in March 2017 in the hope of attending a Korean language class and working part-time to pay off the VND300 million ($12,950) her family borrowed for her.

She then got married to a Vietnamese classmate in South Korea and they now live in Daegu City.

They plan to have a baby in 2020, when her husband will drop out of school and look for work. He has done two years of university.

"If I am pregnant, my husband will be forced to skip class and find work to save the VND20 million a month tuition fee and earn money to cover living expenses," she says.

"We will stay in South Korea longer to try and earn enough money to pay off the debt and to return home and open a small restaurant."

Trang says it is not difficult for Vietnamese students to find work in South Korea since factories and shops need young workers who work hard.

Therefore, Korean owners do not ask the students for their work permit or medical examination results, and in fact even cover up for them if the police come around, she says.

The pressure to make more money to cover living expenses in an expensive country like South Korea forces many Vietnamese students to work overtime regardless of health risks.

It is 3 p.m. and Minh Quang leaves his Korean class at a university in Busan and hurries to the bus stop to go home and prepare for the night shift.

He says: "The night shift starts at 4 p.m. and ends at 11 p.m. The class ends at 3 p.m., and it takes an hour by bus from school to home, and so I’m always in a hurry for fear of being late for work."

Quang pities himself for working at a restaurant for seven hours a day on weekdays and 12 hours during weekends while his friends hang out, taking photos on the streets.

He has been in Korea for nearly two years under the D4-1 visa program, and is about to finish the Korean language and other related courses, which will qualify him for university.

Students on D4-1 visas can work part-time up to 28 hours a week. Quan says if there are no government scholarships or one does not belong to a rich family, Vietnamese students cannot afford the cost of living in South Korea and are forced to work overtime to earn enough money to cover their expenses.

If he only works the permitted number of hours, he can only earn VND13-15 million ($560-650) a month while his tuition and living expenses amount to 15-16 million, he adds.

International students work extra hours usually with the connivance of employers. To throw dust in authorities’ eyes, the latter pay the contracted wages into a bank account registered with the government and the overtime wages in cash or a different account.

Quan explains that employers are willing to break the law due to the huge demand for labor, especially for young workers who can do heavy manual work.

On December 10 Incheon National University reported to the police that 161 Vietnamese students enrolled in the Korean language program were missing. They were among 1,900 Vietnamese attending a one-year course that began four months ago.

On December 12 Pham Quang Hung, head of the international relations department at Vietnam’s Ministry of Education and Training, said the ministry has joined hands with its Korean counterpart to investigate. It would first check if the students had gone to South Korea through illegal agents, he said.

The South Korean government now plans to tighten the issuance and extension of visas for Vietnamese students, and keep a closer eye on students working part-time.

Illegal aliens are placed in a black list by the Korean immigration office and have little chance to ever enter the country again. Besides they could be fined $5,000 and jailed for up to six months.

Last year 13,945 international students overstayed in South Korea compared to 5,879 in 2015, according to the Ministry of Justice. Two-thirds were Vietnamese, it said.

This prompted South Korea to tighten visa rules in March.

Vietnamese students will now have to deposit at least $10,000 in a Korean bank and can only withdraw up to $4,443 every six months.

Earlier international students were only required to prove they had at least $9,000 in a bank account in their or their parents’ name, but this was changed after many took loans to get the visa and withdrew the money right afterward, South Korea’s justice ministry explained.

The new policies would only apply to Vietnamese students with D-4 visas attending universities that the Ministry of Education has deemed "poorly equipped to handle foreign students," based on certain criteria including the percentage of students who overstay their visas and the percentage of students who fail to graduate.

Vietnamese students are the fastest growing group in South Korean universities, second only to China in enrollment numbers, according to figures from the National Institute for International Education under South Korea's Ministry of Education.

There are currently more than 37,400 Vietnamese students in South Korea, up 10,000 from last year, according to the institute.