In essence, the spat with Zelensky was the result of him constantly expressing his distaste for a truce, which annoyed Trump.
In order to understand the matter, it is not enough to watch the last few minutes, but the entire conversation of 50 minutes.
x.com/collinrugg/status/1895590525454983416
For 40 minutes, everything went more or less normally. Trump talked about ores, minerals and soldiers dying, and Zelensky about guarantees and how Putin must be stopped. Trump repeats several times: first a deal, then we'll go easy on guarantees, Zelensky insists that Putin can't get anything, that he has to pay, that he's a murderer and a terrorist. He lies like a dog about who violated the ceasefire in the last ten years and who used the Minsk agreements for what. He is visibly irritated by the fact that Trump strictly uses the term "President Putin" and is careful not to insult the Russians, and puts them more or less on the same level as the Ukrainians.
It's not a very happy atmosphere, but it goes well for 40 minutes. Trump presents himself as a peacemaker and at one point talks about how the war is terrible and the cities have been destroyed, this fool interrupts him and says without a hint of hesitation: "Perhaps Putin told you that he destroyed our cities, we live, we work, our children go to school" (this is the second time in two days that he publicly accused Trump that Putin is making up arguments for him). Trump passed over at least two or three similar remarks to him and actually behaved quite diplomatically, considering how that jerk performed and behaved.
All hell breaks loose at the moment when the journalist asks a question about Poland like this: "Poland was under Russian control for decades after WWII. They looked at America not only as the most powerful country, but also the richest, with the best music and movies. My friends in Poland are now worried that you are aligning yourself too much with Putin. What is your message to them?"
Trump gives literally the most humane and most polite answer on the entire press conference: "If I didn't agree with both of them, we wouldn't have a deal. I can take it upon myself now to spit on Putin, and what happens next? Should I call him and say hey Vladimir, let's make a deal? It doesn't work like that. I'm on the side of the USA here. I can be the most toughest guy in the world, no problem, but then we don't have a deal."
There, Vance feels the need to add that they had a president who for three years pounded on the chest of a hero, cursed and spat at Putin on a dog sled, and how far did it go? It is time to use diplomacy instead of curses, because this is where the US can truly show its greatness.
This remark of Vance's went to the journalist who asked the question, not at all to Zelensky. And there was absolutely no reason to argue with him.
Zelensky neither left or right, but straight to Vance: "Can I ask you something?" And there he reads him a lesson half of lies and half of delusions, and without an iota of hesitation he ends it with: "So, J-D, what kind of diplomacy are you talking about?"
So, he reads him a lesson (despite there not being any provocation), and in that scrubbing calls him by his name and not "Mr. Vice President". A diplomatic scandal just like that.
Vance lost his cool because he has no experience in these situations and is in a style that no one will do the scrubbing on his turf. This one accepts the argument, and then Trump loses it too and we know the rest.