40 million voters were urged to do their civic duty
About 40 million voters in one of the world's few remaining communist states had to choose 500 MPs from more than 700 candidates.
Turnout in the capital, Hanoi, was nowhere less than 70% by midday and up to 95% in other regions, Vietnamese state television reported.
But the French news agency AFP found few people expressing enthusiasm for the five-yearly exercise in "people's democracy" with many saying a relative had gone to vote for them.
Vietnam is a one-party state and the Communist Party oversees the selection of candidates.
This year it allowed 13 independents onto the final list of candidates, but they still have to accept the party's constitutional monopoly of power.
The general secretary of the Communist Party, Nong Duc Manh, was the first person to vote in his constituency in the capital, Hanoi.
Speaking to reporters afterwards, he reaffirmed his commitment to tackle political corruption - a major issue in the election campaign.
"If a candidate is selected there are responsibilities which must be fulfilled," Mr Manh said. "It's not a position where you can simply sit there and expect to get well paid."
Three Communist Party officials have been disqualified, one of them linked to a major underworld figure, Nam Cam, who is in jail on murder charges.
The BBC's Clare Arthurs in Hanoi said the moral character of candidates is one of the decisive factors in getting through the communist party-controlled selection process.
As they went to vote, people were bombarded with signs reminding them that they had a moral obligation to choose good candidates, to ensure they get better lawmakers.
From 6 a.m, street-corner loudspeakers played patriotic music and policemen urged passers-by to go and exercise their "civic right and duty".
Reform agenda
The assembly will be charged with passing often controversial reforms to integrate Vietnam into the world economy.
Its tenure is expected to include Hanoi's bid to join the World Trade Organisation and implementation of a broad trade pact with the United States.
Analysts say economic opening poses significant obstacles as well as risks for the communist system, given powerful vested interests opposed to reducing protection of state industries.
Since the mid-1980s, the communist authorities have gradually abandoned the planned economy, and last year launched a three-year programme of structural reforms supported by the World Bank and International Monetary Fund.
But political liberalisation remains anathema to a regime which watched the collapse of its eastern bloc sponsors with horror.
One party
Those few European politicians who have dared to raise the issue of multi-party democracy with top officials here have all been politely rebuffed.
Challenging the party's "leading role" remains an imprisonable offence for which dissidents are still periodically detained.
At 16.5%, the proportion of non-party candidates was a slight increase on the outgoing 450-seat National Assembly elected in 1997.
But deputy speaker Mai Thuc Luan said it was no time for any fledgling opposition party to get excited.
This year's vote is taking place on the birth date of the founder of modern Vietnam, the late Ho Chi Minh.
Results are due in a week.