As US vice-president JD Vance entered the fifth hour of negotiations with Iranian leaders over the weekend, US president Donald Trump weighed in with an ill-timed threat to start bombing again.If the Iranians closed the Strait of Hormuz, Trump told a Fox News reporter, the negotiators talking to Vance would never make it back to their country – in fact, they would have no country to return to at all.For Vance, this was the latest example of his increasingly tricky role as the frontman in the US negotiations with Iran, as Trump repeatedly creates disruptions in his path.On Monday, Vance said the first round of talks had laid “a successful foundation” for peace. But now, Vance will have to find a way to end a war that he opposed at the start, while navigating his boss’s whims and an adversary that has proven itself, at least in part, immune to Trump’s threats.“What we told the Iranians yesterday is when you guys engage in what us millennials might call trash talk, you can’t expect the president of the United States not to respond and not to correct the record,” he said on Monday at a news conference. “So when they say things that aren’t true, the president is going to respond to it.”[ Trump’s Iran deal leaves him facing a Carter-like dilemmaOpens in new window ]Both sides have signed a memorandum of understanding to end hostilities and are now trying to strike a lasting nuclear deal in 60 days. But for Vance, the presumptive favourite for the 2028 Republican nomination, the situation remains politically precarious.“If it works out, I’m going to take the credit,” Trump said of the peace deal last week. “If it doesn’t work out, I’m blaming JD.”Vance has said the president was joking, but Trump has never shied away from deflecting blame on to others – and how Vance handles the future of the negotiations will factor into Republicans’ performance in the midterm elections and his future as a potential successor to Trump.US president Donald Trump with vice-president JD Vance and secretary of state Marco Rubio during a meeting in the Oval Office of the White House. Photograph: Doug Mills/The New York Times