He has said thank you, multiple times, and praised the prior administration. This administration has been in office for less than 2 months and hasn't done anything but publicly praise Putin and pretend that Ukraine started the war. Vance was demanding that Zelenskyy say thank you to Trump. For what? He acted like a toddler and he was trying to look tough in front of his boss because he's been mostly excluded from all the Cool Kid MAGA stuff. Trump, when asked last week, wouldn't even say he would support Vance as his "heir apparent."
So lets throw away all my feelings and opinions on the current administration buddying up to Putin and even my supposition on why Vance was behaving that way. Lets focus on the actual issue here: Zelenskyy has thanked America, multiple times, for supporting Ukraine's defense efforts. Vance was demanding Zelenskyy thank Trump, who has been in office for less than 2 months and has objectively done nothing for Ukraine. Even if you believe that Trump is justified in not doing anything for Ukraine, what the hell is he supposed to thank Trump for?