Trafficked into China, many are sold into lives of brutality and horror in Chinese brothels - but what becomes of the rest?
Sadly, these girls are being taken by their own people.
The girls are betrayed by Hmong friends and family members, who sell them out for a share in the profits. Physical abductions are regularly carried out by young Hmong men.
Many of the girls are then sold to Hmong families living in China, who act as middlemen between the Vietnamese Hmong traffickers and the insatiable demand for teenaged girls in China.
The Hmong are a people without a nation of their own, who trace their roots to lands now part of China. They've long been marginalised, both geographically and economically.
Over the course of centuries, facing war and oppression in China, many Hmong people fled south to northern Vietnam, Laos and Myanmar, while others remained behind.
There are now significant Hmong populations along both sides of the border between Vietnam and China. Desperate for opportunities, some have turned to trafficking their own people.
Chinese-Hmong families buy many of the girls trafficked across the border by young Vietnamese-Hmong men.
These families hold the girls captive in their home. A girl may be kept for a matter of weeks or months, and a single family may have half a dozen girls imprisoned at any given time.
Each girl will be stripped of her phone, her clothes, and any identifying documents. She will be given new clothes and often a new haircut.
The girl will be told that she has been sold, and now belongs to the family. Often she will be made to feel guilty for the "hospitality" of the family, and forced to work in their home.
While that may sound strange, it's important to remember that these girls are young, often uneducated, and accustomed to occupying the lowest place in households where older males expect obedience.
A Chinese man in search of a girl will come to the home, choose one, and negotiate a price with the family.
While the original traffickers might receive the local equivalent of US $1,000 for a girl, she may now be sold for US $5,000 or more.
The family will often pass the girl off as a relation - a daughter, or niece - who never learned to speak Mandarin.
The customer will then take possession of the girl and take her to his home, where he will imprison her and treat her as he likes.
If the girl refuses to cooperate with the family, she is often threatened with murder, or sale into prostitution - and these threats may be very real.
While it might seem absurd that a family would take so many risks with a girl only to have her killed, a girls' vagina and uterus are not the only organs of value in China: there also seems to be an active trade in organs for transplants and traditional medicine.
But why are there so many Chinese men wanting to buy girls? Who are these men, and what pushes them to treat other human beings like objects to be bought and sold?I'll be looking more deeply at these issues in the coming weeks - to learn more about this monstrous and largely-ignored issue, subscribe here.
One horrendous story featured recently in the international media is that of a 12-year-old Vietnamese girl who was trafficked and sold to an older Chinese man (variously reported as 35 or 40 years old) who then raped and impregnated her.
Sadly, this is only one of countless thousands of such stories that never received the attention they deserve.
For the past six weeks, we've been raising funds to finish our feature documentary 'Sisters For Sale', and to launch a human trafficking prevention program in high-risk villages of Vietnam.
Over 1,000 people have now contributed a total of $35,316, and I'd like to thank each of you who has now supported the cause.
If you haven't yet contributed, please take a moment to learn why this work is so important for all of us. Check it out now, at sistersforsale.com
We can make a very real difference in the lives of countless young women in danger - but we need your support, and we need it now. Every little bit makes a difference, so please, give whatever you can.
There are less than three weeks to reach our $56,000 target, and for you to get a very special sneak peek at 'Sisters For Sale'. After the campaign ends, the film will no longer be available.
For telling your friends about our campaign, you can win the experience of a lifetime in the mountains of northern Vietnam, including all meals and accommodation for a month - see the details on the page.