Amid the graceful pagodas, temples and French Colonial architecture, the Protestant church in Hanoi is a very ordinary building. The Vietnamese congregation sings enthusiastically, maybe unaware a government official is watching them.
The pastor sits at the back of the church. "I don't have government permission to give an interview," he said, sweat running down his face even though it was a rare cold day in Hanoi. Foreign journalists are accompanied everywhere by government minders and it is danger- ous for Vietnamese to criticise the government, especially during a visit to one of just 300 legal churches that service Vietnam's two million Protestants.
Outside, a young woman led the way to a group of men in the courtyard. "They will only talk if I translate," she said. The men were from the Xao tribe from Lai Chau and Caobang in the far north. They had been converted to Christianity by the neighbouring H'Mong, another ethnic minority who had been Christianised when they fought alongside Americans during the war.
The Xao men had come to worship in the legal Hanoi church and to apply to the government for permission to open a church of their own. The 4,000 Christians in their region are forced to worship in illegal "house churches". Christians, they said, could practise their religion in the cities but in the countryside they were being beaten for their beliefs, and forced to recant Christianity.
A woman who works at the church told us she had to care for injured Christians from Lao Cai and Caobang who come to Hanoi for treatment because local authorities broke their bones or poured boiling water on them for their religion. A farmer said he'd been arrested and beaten by two policemen for five hours last March for converting to Christianity.
Two thirds of Vietnam's Protestants are ethnic minorities, and many live in remote areas where neighbours are sometimes suspicious of the converts. "A lot of people don't understand, so they say he who believes in Jesus Christ is a follower of America and foreigners," said a man called Cao.
Thirty years after Vietnam defeated America, the communist government has made its peace with capitalism. Huge propaganda posters featuring party slogans stand alongside gaudy neon billboards advertising cars and French perfume, unexpected in one of the world's four remaining communist countries.
Last year the US became Vietnam's biggest trading partner, giving Hanoi the second-fastest growing economy in Asia. Increased economic liberty has brought the Vietnamese personal freedoms unimaginable a decade ago. With the easing of restrictions, evangelical Protestantism has exploded, and is practised in thousands of illegal "house churches". This is anathema to the communist government who apply strict regulations to "approved" religions.
Government officials fear evangelical Christianity, viewed as "a religion that originates in America", is being used to undermine Communism through peaceful revolution.
Last Easter, thousands of ethnic minority tribes people took part in demonstrations in the Central Highlands. They were protesting against the confiscation of ancestral lands and religious repression. The marches, attended by around 30,000, according to Human Rights Watch, were brutally quashed by the military. Most of the Montagnards - the collective French name for the Highlanders - are Protestants.
Christian groups in America dubbed it the "Easter Massacre", and the US-based Montagnard Foundation claimed 400 Christians were killed.
This figure was furiously denied by the Vietnamese government, which put the death toll at two and called the Montagnard Foundation a terrorist organisation. The government is now convinced the hill tribes, many of whom have been pushed off their land for state coffee plantations, are being incited to fight for an independent state by counter-revolutionary exiles in the US.
"The Montagnards have always been at the bottom of the social structure," said John, an undercover western missionary who has been working in the Highlands for nearly 40 years. "Then along comes the Christian message. For tribal people who have been oppressed by the system. They stand up and say we can't be pushed around. When the systems sees an alternate ideology developing it begins to persecute."
The "Yardies" of the Central Highlands fought alongside Americans during the war, and US veterans have called for America to impose sanctions on Vietnam. One US doctor who worked in the region during the war said: "After the war was over and the communists won, they still needed an enemy and the protestant faith was a convenient enemy. Religion is seen as a tool to interrupt and overthrow the communist system."
The US designated Vietnam a "country of particular concern" over its religious freedom record. The Vietnamese government points to the American's record in Iraq today and Vietnam 30 years ago, to accuse Washington of "the pot calling the kettle black".
But Hanoi might do well to re-read their own recommendations: "Fighting the contagion of Christianity in the minority areas has the opposite effect ... Actually the numbers grow slowly if we have a relaxed policy, and if we crack down hard, Christianity grows faster."
Outside the Hanoi Church the Xao tribesmen were planning to head back to the mountains. "We pray to God to give us aid," said Thang, a farmer, "no foreigner comes and give us food or medicine. Only God helps us."