As President Volodymyr Zelensky of Ukraine headed into a high-stakes meeting in the Oval Office this week, Vice President JD Vance warned him to “behave.” “Mr. President, so long as you behave, I won’t say anything,” Mr. Vance recounted to Fox News in an interview broadcast on Wednesday, adding that the Ukrainian leader chuckled in response. It was not the first time that Mr. Vance had addressed Mr. Zelensky during a critical diplomatic summit with words more commonly spoken to toddlers than heads of state. In February, during a highly contentious Oval Office meeting with President Trump, Mr. Vance blasted...