US officials said this week they would hold a formal signing ceremony for the US-Iran agreement in Geneva, but Iran's foreign ministry cast doubt on that, saying it was unnecessary after both countries' presidents signed the agreement on Wednesday.
Iran had said it was ready to begin technical talks after the two enemies extended a tenuous ceasefire by at least 60 days with the accord.
But the semi-official Tasnim news agency said earlier on Thursday, before Vance's announcement, that Iran's negotiators needed to see signs of implementation of the interim agreement from the United States before the next rounds of peace talks could begin, and that there was no confirmation that its delegation would travel to Geneva.
Vance and the US delegation had been ready to depart as soon as plans for the talks had been finalised, the White House spokesperson said in a statement on Thursday night.
"But the logistics of these negotiations have never been simple or predictable," the statement said.
There was no immediate response from Iran's government.
The diplomatic back-and-forth over the planned ceremony and photo-op adds to the uncertainty over whether a lasting truce can be found to a regional war that has killed at least 7000 people, sent energy prices soaring and shaken global markets.
Israel, which was not included in the peace talks and has distanced itself from the US-Iran accord, continued its fighting against the Iranian-allied Hezbollah militant group in Lebanon, also raising questions about whether the agreement would hold.
In Washington, some of President Donald Trump's Republican allies in Congress questioned whether he had given up too much in order to end the conflict, which is unpopular with most Americans.
Trump previously wrote he would only end the war with Iran's "UNCONDITIONAL SURRENDER," but the memorandum he signed with Iran instead provides relief from economic sanctions, unfreezes assets worth tens of billions of dollars and immediately provides US waivers for Iran to export its oil.
Iran's Supreme Leader Ayatollah Mojtaba Khamenei said Trump had signed the deal "out of desperation" and signalled that upcoming talks over Iran's nuclear program, among Trump's stated reasons for starting the war, would not be easy.
"If the American side wants to be too demanding, we will not accept it," he said in a written message.
The deal gives negotiators 60 days to reach agreement on the status of Iran's nuclear program unless both sides agree to an extension, and set up a $US300 billion ($A427 billion) reconstruction fund for Iran and other financial incentives.
Vance said Washington would also seek to limit Iran's long-range missiles. When the US and Israel launched the war nearly four months ago, Trump said he aimed to destroy Iran's nuclear program to ensure it could never develop nuclear weapons, end Tehran's ability to strike its neighbours, prevent it from supporting allied anti-Israel militants in the region and make it possible for Iranians to topple their theocratic government.
Trump signed the deal with none of those objectives met.
In the agreement, Iran restated its decades-long position that it will not get or develop nuclear weapons, a position doubted by a succession of US presidents. It also agreed to the onsite "down blending" of its highly enriched uranium stockpile and inspections by the International Atomic Energy Agency as a Non-Proliferation Treaty member, rejecting Trump's wish to remove the material from the country.
US officials say the negotiations could still yield a strong agreement on Iran's nuclear program, aiming to exceed one from 2015 between Iran and the US and other countries that Trump tore up in his first term.
But critics say Iran is in a stronger position now, having withstood a superpower attack, exerted control of the Strait of Hormuz and gained valuable waivers to financial sanctions.
Iran has said it will still exert control over Hormuz in partnership with its neighbour Oman across the strait and intends to charge ships fees for services that did not exist before the war, although it says no fees will be charged during the 60-day negotiations period.
Trump said he expects a complete ceasefire on all fronts.
