It was an absolutely fair question. Trump and Vance are saying let’s solve it with diplomacy. Zelenskyy provides facts confirming the impossibility of doing diplomacy in good faith. Agreements don’t have any value when history shows the other party not respecting the agreements. So, “what kind of diplomacy you mean?” is a fair question.
Vance’s answer “I mean the kind of diplomacy that would save your country” is a meaningless bullshit sentence.
He's trying to make the point that they can't talk peace without material guarantees of security from their allies as part of the deal, which guarantees are absent from the White House's agreement, because they just had an agreement without such guarantees shit all over by Putin, so it's, you know, kinda pointless to do that again. It's making concessions on paper for no guarantee of peace, with an adversary that's already broken a similar agreement, leading to this very conflict. Why make concessions with no guarantee of security in return, when there's zero reason to believe Russia will keep their word? He actually manages to get most of that explanation out, in between interruptions and non sequitur digs from the other two.
The difference between this and the more confrontational corrections of Trump's bullshit in similar situations recently, by Macron and Trudeau, is stark. Trump and Vance were primed to pounce.
Vance’s answer “I mean the kind of diplomacy that would save your country” is a meaningless bullshit sentence.