The two leaders were expected to have an informal dinner on Wednesday before more formal talks on Thursday
North Korean leader Kim Jong Un and US President Donald Trump arrived in Hanoi Tuesday, ahead of a second summit closely watched for concrete steps to dismantle Pyongyang's nuclear programme.
After an initial historic meeting in Singapore in June that produced only a vague statement about denuclearisation, analysts say the second date must deliver more in the way of tangible progress.
The summit itself is shrouded in secrecy and there was still no official word even on the venue when Trump touched down in Air Force One late Tuesday. The two leaders were expected to have an informal dinner on Wednesday before more formal talks on Thursday.
Trump made no comment to reporters on his aircraft but had previously tweeted his hopes for a "very productive" meeting, reiterating that North Korea could enjoy economic riches if it gave up its nuclear weapons.
"With complete Denuclearization, North Korea will rapidly become an Economic Powerhouse," tweeted Trump. "Without it, just more of the same."
"Chairman Kim will make a wise decision!"
Relations between the two mercurial leaders have undergone a dramatic turnaround, from flinging personal insults and threats of destruction to Trump declaring he had fallen "in love" with Kim through an exchange of letters.
But many North Korea watchers dismissed the Singapore summit as political theatre that failed to produce a concrete roadmap to denuclearisation and stressed the Hanoi meeting must deliver more.
"The window for diplomatic progress with North Korea will not remain open indefinitely. The second summit must emphasise substance over pageantry," said Kelsey Davenport from the Arms Control Association.
Even so, Trump appeared to lower US demands for Pyongyang in the run-up to the summit, repeatedly saying there was no rush to rid the North of its arsenal as long as missile and nuclear tests stopped.
"I don't want to rush anybody. I just don't want testing. As long as there's no testing, we're happy," Trump said at the weekend.
He also hinted more summits could follow the Hanoi meeting, reducing expectations of a dramatic breakthrough in the Vietnamese capital.
'Most memorable' moment
In contrast to Trump's direct route to the summit, Kim embarked on a two-and-a-half day, 4,000km odyssey through China by train.
He was greeted at the normally sleepy Vietnamese border station of Dong Dang early Tuesday by a military guard of honour in pristine white uniforms, who presented arms as he strolled down the red carpet waving and grinning, surrounded by a phalanx of aides and security personnel.
Student Nguyen Thu Uyen who handed Kim flowers on his arrival said meeting the leader was her "most memorable" moment.
"Kim Jong Un is very friendly and especially attractive," she said, after Kim became the first North Korean leader to visit fellow one-party state Vietnam since his grandfather Kim Il Sung in 1964.
Kim was greeted in Hanoi by cheering crowds and after a few hours' rest at his hotel, made a 50-minute jaunt to the North Korean embassy - echoing the summit in Singapore, where he took in the sights on an unscheduled sortie.
No shared agreement
Diplomatic progress since Singapore has stalled over the definition of denuclearisation, with Stephen Biegun, the US special representative for North Korea, admitting there was no "shared agreement" of what that means.
The United States has repeatedly demanded the North give up its nuclear arsenal in a final, fully verifiable way.
But Pyongyang sees denuclearisation more broadly, seeking an end to sanctions and what it sees as US threats - usually taken to include the American military presence in South Korea, and sometimes in the wider region.
Pyongyang insists it has already taken major steps, by not testing ballistic missiles or nuclear weapons for more than a year, and blowing up the entrances to its atomic test site.
The North also wants increased security guarantees, which could come in the form of a declaration of an end to the 1950-53 Korean War - that culminated with an armistice instead of a full peace treaty - or opening liaison offices.
Opening liaison offices would signal the first stage of normalising US-North Korean relations, said Go Myong-hyun of the Asan Institute of Policy Studies, and would be an ideal "politically symbolic step" rather than prematurely agreeing to sanctions relief.
Harry Kazianis of the Center for the National Interest said: "Nothing would be worse than for either side to come out of the meeting as if it was a waste of time."