Tran Van Tinh, 41, had been living alone since his wife left him, and established a close relationship with a neighboring family in the central province.
Between January and July 2016 he raped the girl on five occasions when her parents were away.
After she was raped a fifth time, the girl, whose identity has not been revealed, told her parents who complained to the police.
Official data on sexual harassment is not regularly published in Vietnam. But out of 1,600 reported cases of child abuse last year, 80 percent involved sexual assault and in most cases involved someone close to the children or having authority over them, such as neighbors, relatives or teachers.