Vietnam has been revealed to be the third-highest global executioner but details of who is on death row and why remains as elusive as ever.

Hidden behind the term "state secrets", Vietnam carries out lethal injections with an unknown cocktail of homemade drugs, while plans to build five new execution centres have stoked fears more deaths are to come.

In January 2008 the bloodied bodies of two well-liked young female postal workers were found in their office in the rural district of Long An in the Mekong Delta region of southern Vietnam.

One was knocked out with a cutting board and had her throat slit, the other was hit with a chair before suffering the same fate, police said.

Sixty-eight days later, recent college graduate Ho Duy Hai was brought in for questioning.

For six months, the family and lawyers were blocked from seeing the 23-year-old.

When they did, Hai had lost 9kg and was too scared to say anything more than "I'm in pain", his aunt Nguyen Thi Ruoi says.

Hai was given just two 15-minute sessions to speak to his legal team before trial.

Nguyen Van Chuong claims to have been beaten and tortured to force a confession (Image: VietnamNet)

At trial, as Hai proclaimed innocence, his defence pointed out dozens of prosecutorial mistakes and instances of police misconduct.

There was no physical evidence placing him at the scene and the so-called murder weapon had been bought at a nearby market after the murders had taken place, his lawyers said.

Nevertheless, Hai was sentenced to death on December 1, 2008.

Following tireless lobbying from Hai's family, a day before his scheduled execution in 2014 President Truong Tan Sang ordered a review.

The following year, then-deputy chair of the National Assembly's Judicial Committee, Le Thi Nga, found there were "serious violations" within the investigation and the court's ruling was inconsistent with evidence, the Vietnamese newspaper Nguoi Lao Dong reported.

Nevertheless, Hai remains on death row.

Ho Duy Hai's mother, Nguyen Thi Loan and Nguyen Van Chuong's father (Image: Supplied)

"He is from a poor family, his parents divorced when his sister was five years old so it would be easier to target him and make him a scapegoat," Mrs Ruoi said.

Long Trinh, activist and editor-in-chief of the legal affairs website Luat Khoa, said the case highlights the "most concerning" aspect of death row in Vietnam: courts "can put people on death row without any legal basis".

Vietnam continues to classify figures on the death penalty as state secrets.

What is known is that 18 offences still carry death - including drugs, murder and 'threats against national security'.

"On the basis of these vaguely worded national security provisions, Vietnam can kill people for expressing alternative political views," Vietnam Committee on Human Rights (VCHR) Vice President Penelope Faulkner said.

Organisations like hers have historically relied on monitoring local media to piece together what is going on.

A leaked government report from the Ministry of Public Security earlier this year revealed it was worse than feared.

According to the January 2017 report, 429 people had been executed between August 2013 and June 2016. As of July 2016, 681 people remained on death row.

No explanation was provided as to why people were executed.

Nguyen Van Chuong's family are campaigning for a sentence review (Image: Change.org)

In their annual report on the death penalty, Amnesty International last month placed Vietnam as the third largest executor in the world, behind China and Iran.

With overcrowded prisons, a government plan to build five new execution facilities and the creation of locally produced drugs for lethal injections, Ms Faulkner fears Vietnam's execution numbers are set to rise.

"There is no real information or control of these local poisons. The first time they were used on a person he took two hours to die," Ms Faulkner said.

Hope is what Hai's family clings to now.

Last year two men on death row, 80-year-old Tran Van Them after 43 years for murder and 57-year-old Han Duc Long cleared of murder and rape after spending 11 years on death row.

Numerous attempts to contact the Vietnam government have been made by AAP.