The 25 men and 24 women are from northern mountainous provinces such as Lang Son, Cao Bang, Bac Kan and Tuyen Quang along the border between the two nations, and southern Hau Giang Province.
They had reportedly crossed over into China to seek work and were detained by the police for failing to show valid work permits or personal identification papers.
After more than a month in detention they were sent back to Vietnam through the Huu Nghi International Border Gate in Lang Son Province.
In recent years the number of Vietnamese going to China illegally to work, mainly from northern provinces, has remained high. Last year more than 35,000 were caught trying to enter illegally.