He said to Zelensky:
"Have you said ‘thank you’ once this entire meeting? No. In this entire meeting, have you said ‘thank you’? You went to Pennsylvania and campaigned for the opposition in October. Offer some words of appreciation for the United States of America and the president who is trying to save your country."
I just rewatched the video. Zelensky said thanks or thank you at least four times at the meeting before Vance starting berating him. I guess JD just wasn't listening and was fantasizing about what he could do with that nice couch.
As for Zelensky "campaigning for the opposition [Harris] in October," NBC reported that it never happened. He did visit an ammunition factory in Pennsylvania, but it was strictly apolitical with no campaign personnel present.