As he defended the US-Iran deal at the White House press briefing on Thursday, Vice-President JD Vance brushed aside a question about whether President Donald Trump had positioned him as the "fall guy" for an agreement that is broadly unpopular with Republicans in Washington.

"I think the president was joking," Vance said, referring to Trump's comment the previous day that he might blame the vice-president if the deal collapses.

Vance has spent the week defending the memorandum of understanding with Iran.

Yet he was often contradicted or overshadowed by Trump - and his uncertainty about the logistics of a signing ceremony he was planning to attend in Switzerland with Iranian leaders only further underscored his challenges in handling a defining issue of his vice-presidency.

Late Thursday, the White House announced that Vance would ultimately not be travelling for the ceremony, at least for now.

Despite all the challenges, Vance has still delivered a forceful defence of the deal.

He also delivered a blunt rebuke of Israel's response to the agreement, going further in his criticisms than anything Trump has said in recent days.

The timing is awkward for Vance, who just days ago published a memoir that intensified speculation about a possible presidential run in 2028.

It will be hard for him to sell this interim deal to a party that is divided between anti-interventionist Maga supporters who opposed the war from the start, and conservative Iran hawks who believe the White House has ultimately capitulated to Tehran.

Other senior administration officials, meanwhile, are not facing quite the same pressures as Vance when it comes to Iran.

Secretary of State Marco Rubio, a potential rival for the 2028 Republican nomination, has manoeuvered himself out of the spotlight on this war.

Secretary of Defense Pete Hegseth has been a vocal defender of the military campaign, but has not been deeply involved in the diplomatic talks to end it and is not the face of this agreement in the same way Vance is.

Some Republicans said Vance's Iran war portfolio has turned into a thankless assignment from a president long known for blaming his subordinates for unpopular policies.

"It's not in the president's nature to cede the limelight and he's done that here," said Matt Mackowiak, a Republican strategist. "That does feel like a deliberate choice."

A longtime Republican operative and critic of the president said Trump was pinning the deal on Vance.

"It's classic Trump to throw JD under the bus," the source, who asked not to be named, said.

It is certainly not a foregone conclusion that Vance's ties to the Iran war will backfire politically.

If the countries reach a final peace agreement that limits Iran's nuclear programme, Vance will have played a central role in achieving a longtime goal of the US and its allies in the Middle East.

But there is no guarantee the two sides will broker a final deal on various deeply technical issues over the next 60 days, or that a long-term agreement would satisfy critics at home and abroad.

"Vance being connected to the Iran war is one more way [that critics will] hold him accountable for Trumpism," said Terry Holt, a veteran Republican consultant.

The week of mixed messages on this deal illustrated Vance's challenge on Iran.

The administration announced that Iran had agreed to the memorandum of understanding on Sunday, but did not release details.

The lack of clarity sparked confusion about what was actually in the text of the deal. Vance then tried to clear up the confusion in several interviews.

He told CBS News on Monday that Iran "could have access" to a $300bn reconstruction fund if it abided by the terms of its deal with the US.

Hours later, Trump said in a social media post that reports of the US paying Iran as part of the $300bn fund were "Fake News" and told reporters "we're not putting up 10 cents".

When the text of the agreement was finally released by US officials, it included a provision committing the US to work "with regional partners to develop a definitive mutually agreed plan with at least USD $300 billion" for Iran's reconstruction.

On Iran's nuclear programme, Vance echoed Trump's assertion that the interim agreement was a significant first step toward stopping Iran from developing nuclear weapons.

But the deal did not address the nuclear question in detail, leaving that to be settled in the next round of talks.

Throughout the week, Vance was also promoting his new book about his faith and conversion to Catholicism.

But even in those media appearances he could not escape the news of the day.

As he was sparring with Whoopi Goldberg on ABC's The View, Trump was meeting with world leaders at the G7 summit in the French resort town of Évian-les-Bains.

At his press conference on Wednesday, Trump reprised the joke he has made in recent months that he would blame Vance if the Iran deal fails.

Trump also appeared to downplay the significance of the memorandum of understanding, at one point questioning whether it was an important enough document for him to sign.

Then, soon afterwards, Trump signed a paper copy of the deal on camera during a lavish dinner with French President Emmaneul Macron at the Palace of Versailles - raising questions about why Vance would then need to stage a separate signing event of his own in Geneva.

A question that was answered later when the White House announced Vance would not be travelling there - at least for now as logistics for talks with Iran were not yet finalised.

With Trump still out of the country, Vance had continued defending the deal as his former colleagues in the Senate criticised it for giving up too much to Iran.

"Iran's nuclear ambitions were not curbed, and they have learned that threatening the Strait of Hormuz works and will undoubtedly leverage it in the future," Republican Senator Bill Cassidy of Louisiana said in a post on X.

Senate Armed Services chairman Roger Wicker of Mississippi said on Thursday that the deal was "completely out of step with the president's goals".

Vance pointed to falling petrol prices at the press briefing on Thursday, arguing that the deal was already paying dividends to Americans.

He expressed confidence that it would continue to bear fruit so long as Iran made good on its promises and agreed to a final peace deal once the key issues have been negotiated.

"If they change their behaviour, big things are going to happen," Vance said. "If they don't, no skin off our backs. Either way, we win."

With Vance leading these negotiations, as Trump made clear, he will have a lot riding on a victory - or a defeat.