him about complete denuclearization and peace and i think president trump sees himself on the cusp of a great historical accomplishment. >> reporter: tonight, as kim moves toward vietnam in his heavily armored train, there's enormous pressure on both leaders to come up with a more substantive deal than the vague agreement they struck in singapore more than eight months ago. >> it's a test of kim jong-un's sincerity and something that's the most important indicator to watch for in the summit. >> reporter: mark lippert knows sting about how difficult it is to reach a deal. lippert put his life on the line working toward peace between the two koreas and a u.s. >> i need an ambulance fast, get me to the hospital. >> reporter: in 2015, a south korean extremist stabbed him in the face at an event in seoul while lippert served as u.s. ambassador to south korea. he believes a successful summit would be for trump to get kim to make specific commitments. >> i thnk atink that at a minim would start to define future activities.