From the "no deal" outcome of the highly-anticipated nuclear summit to the North Korean leader responding earnestly to a question from a foreign journalist for the first time, there were plenty of subtle takeaways from high-profile meeting between Donald Trump and Kim Jong-un in Vietnam this week.

While the second summit collapsed after two sides failed to reach a deal due to a stand-off over US sanctions, experts maintain that many lessons have been learnt by both sides.

Here is a list of some of those lessons from the experts themselves about how they think the summit went and whether they thought it was a success or a failure.

Rebecca Hersman — expert in nuclear issues at CSIS

"The Hanoi Summit outcome is disappointing but not surprising. Top down, summit-driven diplomacy was worth a try. It's a viable strategy. And no question — it's better for the President to walk away from a bad deal and regroup going forward. This is a setback but not a reason to terminate diplomacy — it's more important than ever. But I think the President only gets one walkaway and now he has used it. For any future meeting the ink needs to be nearly dry on something — even an interim or incremental something — before they meet again. Of course, a comprehensive approach that accounts for the full and verifiable denuclearisation of the DPRK is preferred, but not realistic. "Bottom line: Kim Jong-un once again walks away the winner here. He was yet again given a stage with the United States President who speaks warmly of him and his leadership and even provides him cover for the death of a United States citizen who died as a result of his illegal detention in a North Korean prison. Kim Jong-un even took his first question from the western press and everyone threw a party. So the question is how can the United States incentivise real concessions from North Korea without risking another dangerous round of provocation and crisis like we observed in 2017?"

In-Bum Chun — former South Korean lieutenant general

Sorry, this video has expired In-Bum Chun on Donald Trump and Kim Jong-un's Vietnam summit

"No deal is better than a bad deal. I think there is a miscalculation on the part of the North Koreans that they thought that because of President Trump's internal political issues he would be easy to make a deal with in Hanoi, and they would be able to get a comprehensive deal which is the closure of the Yongbyon [nuclear facility] and immediate sanctions relief. And of course what has happened is that the United States has asked — 'Ok you can close the facilities at Yongbyon, but we want to see it and we'll verify it and then we'll lift some of the sanctions' — and I think that's where it must have [broken down]. "I hope the North Koreans really learned their lessons about democratic nations — that even if you are the President of the United States, you can't make a deal on a whim."

Jean H Lee — former Pyongyang AP bureau chief

Jean H Lee is director of the Hyundai Motor-Korea Foundation Centre for Korean History and Public Policy at the Woodrow Wilson Centre. ( Supplied )

"While it's not uncommon for negotiators to walk away from high-stakes negotiations, it's unusual for this to happen at such a high-profile top-level summit that had been touted by both sides, with much fanfare, as a historic occasion. The dramatic rupture underlines just how difficult these negotiations are, despite the cavalier way President Trump declared the North Korea nuclear threat 'over' last year. It was a gamble to leave negotiating teams so little time to carry out the tough work it takes to agree on a roadmap to denuclearisation. And it was risky to continue those negotiations into the summit instead of calling them off beforehand. "No matter how warm the unusual relationship between Trump and Kim, the power of their personalities is not enough to resolve a standoff that has gone back 70 years. President Trump clearly sees Kim as a 'little brother' that he can bully into good behaviour. It's a patronising approach that underestimates how stubborn the North Koreans can be, especially when it comes to giving up their prized nuclear program. "As for Kim, I'm deeply disappointed in his decision-making. He absolutely needs to give up core elements of his nuclear program in exchange for concessions. Only when he makes these concrete moves will he progress down the path toward diplomatic and economic engagement with the United States, South Korea and other nations poised to step in with economic help in exchange for denuclearisation. Until then, he continues to hold not only the region but also his own impoverished people hostage to a destabilising and costly nuclear program."

Sorry, this video has expired Donald Trump says North Korea wanted all sanctions dropped in exchange for denuclearisation.

Zhiqun Zhu — political science professor at Bucknell Uni

"Going forward, they must be more flexible and move in tandem. The United States may consider partially lifting sanctions or setting up a liaison office in Pyongyang in exchange for North Korea's concrete steps to dismantle Yongbyon and other nuclear facilities. The wellbeing of the North Korean people should be the priority. While keeping the pressures on, the United States must plan to help North Korea realise its economic potentials. "It's probably the first time Kim has answered a question from a Western journalist — I'm not sure whether he answered any questions from journalists in China or Korea in the past. Regardless, it shows Kim's growing confidence and openness. His answer appeared frank and appropriate. He was also obviously serious about North Korea's denuclearisation. As he remarked, if he were not serious, he would not have come to Hanoi. So I think it is a positive sign, and the outside world should encourage Kim to open up North Korea and help him implement his economic development policies."

Christine Ahn — executive director of Women Cross DMZ

"This is a huge missed opportunity and a disappointment to millions of Koreans … but transforming 70 years of enmity doesn't happen overnight. Trump and Kim have clearly made strides in building trust, as exemplified by Trump's acknowledgement of the harmful effects of sanctions on the North Korean people and the cost of the US-Republic of Korea 'war games'. We believe the groundwork has been laid. "Going forward, there must be a more robust official diplomatic process, as well as the inclusion of civil society, especially women, that have on-the-ground expertise and insight that can help both governments come to greater understanding. "I thought it was brilliant that the North Korean delegation held a press conference last night to counter Trump's distortion about the breakdown in talks. Between Kim Jong-un's response to the media and this press conference, it signals to me the Government is more willing to engage on the international stage. That's a positive sign and I'm encouraged that both sides — though they walked away this round — have kept the door open for more."

Sorry, this video has expired North Korea's foreign minister comments on meeting

Jie Chen — political science expert from the University of WA

"President Trump probably assumed that he could just use personal touch to do a deal with the world's most reclusive leader [which decades of hard diplomacy had failed to resolve]. Apparently the summit failed because Kim Jong-un asked for too much by giving away too little. Such bottom-line differences should have been ironed out beforehand. "We should appreciate what has been achieved by the Trump-Kim interactions since early 2018: there has been no test of rockets or missiles, no bellicose diplomatic gesture. Let's hope that within the overall dilemma moderate advances can be made, such as opening up liaison office, pending more substantial progress on the real issue which I suspect won't happen any time soon."