Nguyen Thi Thuy, 32, was taken in for questioning after her human cargo of 32 workers was found on June 9.
Police have opened a case to investigate her for sending people to work in China illegally.
They were on a bus to Lang Son Province to cross the border.
Eighteen of these people were returning to China after a home visit while the rest were going for the first time.
The workers look for jobs on their own.
It is not known how much Thuy charged each for transporting them.