For 39 days, Vice President JD Vance performed his duties in the shadows of two bigger-than-life figures: President Trump and Elon Musk.

That changed on Friday.

With cameras rolling, the vice president ambushed President Volodymyr Zelensky of Ukraine, triggering the kind of overheated argument rarely seen in the Oval Office. Mr. Vance repeatedly accused Mr. Zelensky of disrespecting Mr. Trump by refusing to offer thanks for U.S. assistance.

“Do you think that it’s respectful to come to the Oval Office of the United States of America and attack the administration that is trying to prevent the destruction of your country?” Mr. Vance asked, yelling over Mr. Zelensky.

It was Mr. Vance’s most high-profile moment since assuming the role of Mr. Trump’s understudy. And it suggested that the 40-year-old, former first-term senator from Ohio is trying not to be relegated to the B-team of what has already become one of the most fast-paced and aggressive administrations in modern history.

The remarkable scene of a vice president injecting himself into the middle of a tense diplomatic discussion in the Oval Office also showcased Mr. Vance’s media savvy. A onetime best-selling author and CNN political contributor, the vice president has demonstrated a knack for seizing on moments that will capture the media’s attention.

On Friday, Mr. Vance found that moment as Mr. Zelensky tried to explain the ways in which President Vladimir V. Putin of Russia had reneged on prior diplomatic deals. Rather than engage with the Ukraine leader on the substance of that question, the vice president used a tried-and-true debater’s technique: He changed the subject.