The Vietnam National Administration of Tourism has ordered travel companies to thoroughly check personal information and background of tourists before sending them abroad, after 56 Vietnamese disappeared during their visit to South Korea last week.
The missing tourists, of whom 19 have been found, were the clients of four travel companies in Hanoi. They have been suspected of trying to find jobs illegally in South Korea.
The group came to the resort island of Jeju, about 120 kilometers from South Korea's coast, on a charter flight last Tuesday with another 99 Vietnamese. They disappeared the next day, according to media reports.
Tourists have been allowed to enter Jeju and stay there for 30 days without visa requirements since 2011.
Ten tourists were caught earlier last week and are under investigation, according to the local immigration office.
Another nine were found on Saturday, three of whom were caught working at a workshop and the rest at a rented house, news website Jejusori reported on January 16.
The immigration office said it is investigating if the Vietnamese tourists were trying to look for illegal employment in South Korea, Yonhap reported.