By Clare Harkey BBC East Asia regional editor

The scandal has dominated headlines in Vietnam

The scandal centres around a transport ministry project management bureau known as PMU-18 - the government's richest state agency.

It manages around $2bn for road construction and other infrastructure projects, using government money as well as funds from overseas donors, notably Japan, the European Union, Australia and the World Bank.

The first sign that something was seriously wrong in PMU-18 came when the head of the bureau was arrested in January and accused of taking around $7m of the unit's budget for gambling on European football matches.

He is also under investigation over the procurement of luxury cars for other government officials.

Deputy Transport Minister Nguyen Viet Tien - a former head of PMU-18 and also a member of the ruling Communist Party's Central Committee - was next to be detained. And earlier this month, the transport minister himself, Dao Dinh Binh, resigned.

It seemed he was pushed into the move by high-ranking Party officials.

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Vietnamese press reports say it is also beginning to look as if the PMU-18 network extends into the office of Prime Minister Phan Van Khai, and into senior police ranks.

In recent years the government has taken action against corruption, making officials declare their income and passing laws against money-laundering.

The authorities have promised that this scandal will be fully investigated too.

And they have promised to put the issue at the top of the congress agenda.

But it is a delicate balancing act - scandals like this undermine public confidence in the authorities and risks precious foreign aid.