Six world powers and Iran are reported to have drawn up a 100-page document detailing curbs to Iran's nuclear activity and sanctions relief.
The sides have held multiple rounds of talks to try to end the 13-year crisis.
The world powers suspect Iran of seeking to develop nuclear weapons - something it strongly denies.
Unnamed diplomats at the talks told the Associated Press news agency that negotiators planned to announce on Monday that they had reached a final accord.
The BBC's James Robbins in Vienna says the text of the document is long and the language apparently very precise, to minimise the risk either side can question later the commitments they have made.
Negotiators at the talks between the so-called P5+1 - the US, UK, France, Russia, China plus Germany - and Iran have been struggling over the issues of international inspections of Iran's non-nuclear sites, verification of Iranian compliance, and the pace of sanctions relief.
Officials from the two sides expressed cautious optimism on Sunday that they were close to a deal.
US Secretary of State John Kerry said "a few tough things'' needed to be resolved but added: "We're getting to some real decisions."
French Foreign Minister Laurent Fabius, who cancelled a trip to Africa to stay at the talks, said: "I hope we are finally entering the final phase of these marathon negotiations. I believe it."
In Iran, President Hassan Rouhani said the sides had "come a long way". "We need to reach a peak and we're very close," the Isna news agency quoted him as saying.
If a long-term agreement is reached, it still faces a tough ride through the US Congress, which could reject it and leave US sanctions against Iran in place.
Mitch McConnell, the top Republican in the Senate, told Fox News Sunday: "I think it's going to be a very hard sell... We already know it's going to leave Iran as a threshold nuclear state."
A deal is also strongly opposed by the two main US allies in the Middle East, Israel and Saudi Arabia, who feel threatened by Iran.
Israeli Prime Minister Benjamin Netanyahu on Sunday accused the P5+1 of "caving in" to Iran. It followed comments by Iran's Supreme Leader, Ayatollah Ali Khamenei, who said the fight against US "arrogance" would continue regardless of the outcome of the talks.
There are growing expectations that a comprehensive deal capping marathon negotiations over Iran's nuclear programme will be announced in Vienna.