Trong, 74, was the only candidate on the paper for the powerless National Assembly a month after the former president Tran Dai Quang died after a prolonged illness. The 61-year-old former police chief had been president since 2016 and was associated with a crackdown on freedom of speech.
Trong will remain party general secretary, making him the first figure to hold both roles since war leader Ho Chi Minh in the late 1960s.
The swearing-in ceremony saw 477 National Assembly members back Trong, with one vote against.
In an uninspiring acceptance speech at the rubber-stamp parliament, Trong asked for sympathy from the nation, mentioning his “worrisome” age and health.
“This is a very huge honour, while at the same time, a very heavy responsibility for me … My qualifications, capacity and my limits are obvious, my knowledge is not sufficient,” Trong told parliamentarians. “I’m getting old and my health is becoming weaker, so it should be worrisome. Therefore, I’m asking for sympathy and help from all of you and from the legislators, from the government administration, from the Vietnamese people.”
Trong referred to the difficulties of maintaining Vietnam’s position as one of the world’s fastest-growing economies.
“We have a lot of heavy tasks awaiting ahead to be done as international developments are so unpredictable,” Trong said. “We must be very careful.”
He has welcomed a US role in the region to offset Chinese expansionism. Beijing claims more than 80 per cent of the South China Sea, based on a 1947 map showing an imprecise “nine-dash line”, significantly overlapping with what Hanoi regards as its territorial waters.
Trong’s dual role in two of Vietnam’s four leadership positions breaks with Vietnamese tradition and gives him considerable power in the secretive government.
The two other leadership roles are the prime minister and National Assembly chair.
Observers are expecting a renewed focus on Trong’s anti-corruption campaign which he has promoted since his re-election as party boss in 2016.
The campaign has seen numerous civil service chiefs, business leaders and bankers jailed, with many seen as Trong’s political opponents after because being associated with former prime minister Nguyen Tan Dung, who was sidelined in the 2016 reshuffle.
More than 55 dissidents have reportedly been jailed this year alone.
Nguyen Phu Trong with US Defence Secretary James Mattis. Picture credit: US Department of Defence