Karadzic faces 11 charges against him at The Hague tribunal, including genocide and conspiracy to commit genocide for allegedly masterminding the 1995 slaughter of 8,000 Muslim men and boys in Srebrenica - Europe's worst massacre since World War II.
A prosecutor interviewed Karadzic for more than an hour on Friday about the details of the war crime suspect's arrest, Vujacic said.
Karadzic then had until midnight to lodge the formal appeal, Serbian court spokeswoman Ivana Ramic told The Associated Press.
Once the Serb court receives the appeal, a panel of judges will meet to decide on it, Ramic explained. After that, the case will be handed over to the Serbian government, which issues the final extradition order.
But all Vujacic would say was that he planned to mail Karadzic's appeal to the court five minutes before post offices closed at 8 p.m. (1800 GMT) - a move aimed at prolonging Karadzic's extradition period. He predicted that Karadzic would not be extradited before Wednesday.
Vujacic later told the AP that he would not confirm filing the appeal, which he said can be mailed from any post office in Serbia.
"All those waiting for me to show up will be disappointed. Any of my assistants or couriers can mail it," Vujacic said. Serbia's state Tanjug news agency quoting him as saying this was part of his defense strategy.
The lawyer could not be reached for comment after the post offices closed.
Karadzic had been a fugitive for over a decade before he was arrested. Government officials say he was captured Monday, while Vujacic claims Karadzic was apprehended last week and held incommunicado by unknown kidnappers for three days.
Vujacic has filed a lawsuit against Karadzic's alleged abductors. Responding to the lawsuit, a prosecutor spoke to Karadzic about the claims Friday, Vujacic said. Court spokeswoman Ramic said "all important circumstances" are being taken into account in the extradition procedure.
Vujacic also says his client plans to defend himself against U.N. genocide charges, just as his mentor, former Yugoslav President Slobodan Milosevic, did. Milosevic died in 2006 while being tried for genocide.
Karadzic's arrest has sparked Serb nationalist anger.
Several hundred ultranationalists - chanting Karadzic's name and denouncing Serbian President Boris Tadic - marched Friday for the third straight day of protests in downtown Belgrade in support of Karadzic.
The demonstrators briefly scuffled with riot police and hurled burning torches at the Belgrade City Council building. They did not follow the lead of Thursday's protesters and also attack reporters covering the demonstration.
Vjerica Radeta, a top official and lawmaker from the Serbian Radical Party, warned the pro-Western Tadic. Radeta said Tadic may meet a fate similar to Zoran Djindjic, the Serbian reformist prime minister assassinated in Belgrade in 2003 by nationalists opposed to his extradition of Milosevic to The Hague.
"We remind Tadic that treason has never been forgiven in Serbia," Radeta said. "Every traitor in Serbian history has met with damnation."
Also Friday, more possible news on Karadzic's nearly 13 years under cover emerged.
Austrian police Col. Rudolf Golia told the AP that anti-terror units found a man who looked exactly like the fugitive Karadzic's assumed identity while searching an apartment in Vienna for a murder suspect last year. The man was not connected to the killing, and he was let go without being fingerprinted.
Meanwhile, an Austrian newspaper reported that Karadzic worked in Vienna as a "miracle healer," seeing patients in homes of Serbians living there. The Kurier newspaper quoted a married couple who said they sought his services after trying in vain to have children. Their encounter with him occurred in mid-2006.
In Serbia, the Vecernje Novosti daily reported that Karadzic has been reading the Bible since in detention, drinking only water and eating whole-grain bread.
The fugitive Karadzic had assumed a false identity of "Dragan Dabic." The real Dabic is a 66-year-old construction worker from Ruma, a town north of Belgrade, government official Rasim Ljajic confirmed, adding that Dabic's ID differed from Karadzic's "only in the photographs."
