"Hi, how are you? Where are you from?"

A boy has stopped a middle-aged man on a busy walkway beside Hoan Kiem Lake, one of central Hanoi's busiest tourist attractions.

It's about 8:00pm and my fixer and I are sitting nearby, researching a story on Hanoi's runaway street boys. There are thousands of kids surviving on Hanoi's streets. Vulnerable and alone, many fall prey to exploitation, trafficking, criminal gangs or long-term abuse by foreign and local paedophiles.

Is this boy one of them?

We watch as the man, short, squat and slightly fox-like, replies. He's from France. It's his first time in Hanoi.

"Oh. Are you here with friends?"

The man says he's travelling alone. "What about you — are your parents with you? Where do you live?"

"Close to here."

It's a popular tourist spot, but Hoan Kiem Lake also has a seedy side. ( Reuters/Kham )

"Oh, well my hotel is nearby too …"

"Oh, really?"

It feels like they are playing a game, sussing out if they both know what they are really talking about.

Eventually the boy asks the man if he would like to "find somewhere to sit down?"

They walk over to a park bench. A woman comes up with a little girl. She starts talking, pushing both children closer to the man, gesturing to the boy as though showing him off.

After a while, she notices us and sends the little girl over. She could only be three or four. She has clearly been given a script and beyond that she doesn't know what to say.

Meanwhile, my fixer walks towards the woman on the pretence of using a rubbish bin. As she passes, the man pays the woman and leaves with the boy.

Two street boys so exhausted that they have simply fallen asleep where they are sitting. ( Supplied: Do Duy Vi/Blue Dragon )

Runaways are easy prey

It was just too easy. Looking back, the woman probably mistook our observation for interest in the little girl. Maybe she was feeling lucky — two quick transactions then she could sell the children again a few hours later. I didn't sleep for several nights after that.

Most of Hanoi's street children, the majority of whom are boys in their pre and early teens, are runaways.

Many were neglected or physically or mentally abused at home. An estimated 71.6 per cent of boys in Vietnam experience violent punishment, often for things as minor as getting poor grades.

Family breakdown has become a serious issue in Vietnam, linked to the country's rapid development, says Michael Brosowski , the founder of Blue Dragon Children's Foundation, an organisation focused on child rights and trafficking victims in Vietnam.

Street kids in a makeshift squatter camp. Some boys sleep there, among the rubble. ( Supplied: Do Duy Vi/Blue Dragon )

"Families in rural areas, where there's less access to opportunities and weaker education systems, don't have the support or the tools to cope," he says.

"It's easier for problems to take hold, like drug addictions or gambling, and families are weakened by parents having to travel far from home for work."

Feeling unloved and alone, the boys run away. But when they get to the city, they are plunged into a hard, cold world where gangs rule, fights over territory are common and everyone is out to get you.

"One particular gang … presents itself as being incredibly wealthy and powerful, with their social media pages full of images of them looking tough and intimidating," says Michael. "This can really appeal to boys who feel disconnected and powerless."

A boy climbs under Long Bien Bridge to relative safety on a pylon right beneath the train tracks. ( Supplied: Do Duy Vi/Blue Dragon )

Sex is a taboo subject

The most significant and traumatic threat the boys face is sexual exploitation.

In one 2013 study with street children in Ho Chi Minh City, 92.5 per cent of participating children reported at least one form of sexual abuse and another 33 per cent had worked as sex workers

In Hanoi, paedophiles wait around bus stations, parks and other places where the boys tend to congregate. Alone and desperate, many boys accept their help. At first, they may not even know they're in danger. Sex is rarely talked about in Vietnam and it's normal for men to share a bed back home.

The paedophiles' trap is simple — they offer the boys money, gifts, safety and protection, food and even emotional and physical affection.

Ta Ngoc Van, chief lawyer at Blue Dragon, says under-aged sex is an under-covered crime.

"We have heard about monks, priests, teachers, singers, friends of families, volunteers at charity organisations. The predator's name only shows up when their crime is discovered."

And then there's expats and sex tourists.

In May, a convicted British paedophile was found pictured with kids in Vietnam. Although there is no suggestion that he has acted inappropriately, it can be easy for foreigners to slip through the cracks. Many schools accept foreign English teachers without vetting them — they may not even have teaching qualifications or work visas

While local predators buy boys for as little as 50,000 dong ($3.20), foreign tourists pay the most — sometimes over 1 million dong ($63).The longer a boy has been in town, the less he charges.

A Blue Dragon team member talks to street boys out in Hanoi. ( Supplied: Blue Dragon )

'Safety' under a railway bridge

Each night, Blue Dragon sends a team out to connect with the boys. I join Step Ahead program leader Do Duy Vi one evening and he shows me some of the places they go for safety.

One hiding place is under the train tracks on Long Bien Bridge. We scrambled down through the rusty steel truss to the pylon below. As Vi jumped down, four used syringes cracked under his feet.

He shows me a jagged hole by one of the rafters where the boys would crawl inside to sleep. It was barely wide enough to fit a person and inside was completely black.

"It looks like this has been taken over by heroin addicts now," he says. "But they used to hide here."

Meth and heroin are cheap, and gangs, pimps or paedophiles may get boys hooked deliberately.

A Blue Dragon Outreach team member chats with a boy who has been living on the streets. ( Supplied: Blue Dragon )

Many boys also rely on gaming addictions, which were often a coping mechanism before they started a life on the streets, says Blue Dragon senior psychologist Dinh Thi Minh Chau.

Gaming cafes become a refuge, both mentally and for physical safety. Boys will stay for as long as they can pay for their seat, gaming all day, eating very little.

"When they play games, they are strong, they have friends, some are good at it," Chau says. "They may be rich in the game world. It's very different to their real lives.

Helping boys escape

When the boys come to the Blue Dragon shelter, a team of psychologists slowly builds trust with them and when they are ready, they start digging deeper. But the process is tough. Vi estimates only about 50 per cent of the boys make it through their program.

Ultimately, the aim is to reconnect the boys with their parents but the family may not want them or be in a position to care for them. Often, the boys don't want to go home.

Confronting their trauma and dealing with Blue Dragon's rules can be daunting, particularly without their usual escapes. Conflict is another issue. The boys have learnt to exist in fight-or-flight mode, so when something happens or someone new arrives, their first response is aggression.

I met one kid who has just run away after stealing and selling a bicycle belonging to an older boy. Vi was trying to coax him back to the shelter.

We met him at a gaming cafe and took him out for a meal. He was tiny — aged 13 but he looked more like 7 — and he was terrified by what the other boy might do if he returned.

It is common for newcomers to run away several times. It can take months for boys to fully trust the team, so Blue Dragon just keeps welcoming them back until they are ready.

Do Duy Vi talking with one of the street boys Blue Dragon works with. ( Supplied: Blue Dragon )

Justice is hard to find

It's often a losing battle to find justice for abused boys. Loopholes persist in Vietnam's Penal Code that make convicting predators incredibly difficult.

Until 2015, the law did not stipulate sexual abuse of young boys as a sex crime. Sex was defined as "penile vaginal penetration".

UNICEF representative in Vietnam Rana Flowers says the new penal code expands the scope of rape and forced sex.

"However, the terms 'sexual intercourse' and 'other sexual activities' remained undefined, thus allowing substantial room for discretion by criminal justice officials," she says.

In April this year, an ex-government official was caught on camera molesting a little girl in a Saigon elevator. His case was first dismissed pending further investigation, but he was eventually convicted last month, although he has said he will appeal.

Barefoot and tired, Hanoi's street boys live life constantly on the edge. ( Supplied: Do Duy Vi/Blue Dragon )

One foreign paedophile, Slovakian national Roman Zmajkovic, operated in Vietnam on and off for seven years. He was finally convicted last year after being caught assaulting a 13-year-old boy and given three years in prison for his crime, followed by deportation. He was also required to give 105 million dong to the boy in compensation.

The boy gave evidence against Zmajkovic, but it's usually very hard to convince the boys to speak up.

"They have their own pride," says Van, the Blue Dragon lawyer. "They feel ashamed. They think it was their fault. They do not want anyone to know, including their parents."

Society is slowly waking up to the realities of child sexual abuse and the Government has been taking steps to better protect Vietnam's children.

"Currently, the Supreme People's Court, with support from UNICEF, is developing a judicial resolution to provide detailed definitions of rape and forced sex, and other forms of sexual abuse," Rana says.

The resolution is due for approval in September.

The authorities have also introduced mandatory sexual assault prevention classes to the first grade curriculum and are training police forces on how to prevent and respond to sexual violence.