Lo Thi Nguyet, 21, was apprehended at the province's border town Dong Dang, and she told the border guards an unidentified person asked her to pick up the seven-day old infant from her mother, 24, along with some related papers.
Nguyet had agreed to take the infant to Fujian Province, China, in return for VND15 million ($650).
Authorities have sent the child to a local shelter and are investigating further.
They said there is no further information yet about the mother.
Last month border guards detained a woman and a man who were allegedly taking a 14-day-old baby to China for sale.
Numerous cases of trafficking of newborns have in fact been reported recently.
Data from the Ministry of Public Security shows that since 2010 alone there have been over 7,000 human trafficking victims, mostly women and children from poor and rural areas.
The ministry said in a report last month that in most cases of human trafficking uncovered since 2016, the victims were sold in China.
Besides the victims’ poverty, experts have also blamed negligence, poor education, weak law enforcement, and gender imbalance in destination countries as major factors.