Vindman believed the existence of a quid pro quo was clear during a July 10 meeting between American and Ukrainian officials.
In a separate meeting of US officials immediately afterward, "Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma," Vindman testified.
But the fact that the $400 million in aid, including desperately needed military assistance, was also being used by the President didn't become clear until the next month, Vindman testified.
Zeldin also questioned why Vindman had not carried out the President's order to push for the Ukrainians to investigate the Bidens.
"I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government's support of Ukraine," Vindman wrote in his opening statement.