He was accused of belonging to the exiled Vietnamese reform party, Viet Tan, which has been denounced by Communist party officials in Hanoi as a terrorist organisation.
Viet Tan has consistently renounced violence. It says it is working to promote the development of civil society in Vietnam in order to create a more pluralistic political system.
In recent years, Vietnam has released other well known political prisoners into enforced exile in the United States.
Observers say that Europe may increasingly provide an alternative location for Vietnamese dissidents.
The European Union had called for the release of Dieu and his fellow activists.
Analysts say the EU may have acquired additional leverage with the Vietnamese authorities now that a free trade agreement between the two sides is due to come into effect.
Vietnam has continued a harsh crackdown on prominent critics of the government, subjecting them to regular harassment, intimidation and in some cases, long prison terms.