* Dollar broadly weak on optimism about vaccines, stimulus
* Euro above $1.21, Aussie just under 26-month high
* Pound near 3-month high, but implied volatilities rise
* Graphic: World FX rates in 2020 https://tmsnrt.rs/2RBWI5E
By Hideyuki Sano
TOKYO, Dec 3 (Reuters) - The dollar slid to a 2-1/2-year low against a basket of major currencies on Thursday as investors wagered that more economic stimulus from Washington and the expected start of COVID-19 vaccinations would support riskier assets.
While U.S. legislators have failed to reach agreement on fresh relief for a pandemic-hit U.S. economy, there were early signs that a $908 billion bipartisan proposal could be gaining traction.
Investors expect lawmakers to reach a deal eventually with the two parties also facing a Dec. 11 deadline to pass a $1.4 trillion budget or risk a shutdown of the government.
Britain on Wednesday approved a COVID-19 vaccine developed by Pfizer and BioNTech and said it would start vaccinating those most at risk early next week.
"Britain is starting vaccination and the U.S. is also expected to do so in coming weeks while coronavirus infections appeared to have peaked in Europe and the same could be said for the U.S. as well," said Yujiro Goto, chief strategist at Nomura Securities.
"All of these are easing worries about the economic recovery losing steam."
That optimism more than offset disappointing U.S. jobs numbers for November and helped boost the euro despite widespread expectations the European Central Bank will enhance its quantitative easing next week.
The common currency ticked up slightly to $1.2119, touching its loftiest level since late April 2018.
The dollar index slipped to 2 1/2-year low of 90.948 and last stood at 90.992.
"On the whole, the new U.S. economic team under President Biden will be dovish, if not directly pursing a weaker dollar per se," said Daisuke Uno, chief strategist at Sumitomo Mitsui Bank.
"The next target for the euro should be its February 2018 peak of $1.2555. When the ECB eases its policy next week as expected, I bet the euro will gain rather than fall," he added.
The U.S. currency was on the back foot against most other currencies, apart from the safe-haven yen, against which the dollar held firmer at 104.47 yen.
The Australian dollar traded at $0.7409, having hit a 26-month high in the previous session after data showed Australia's economy rebounded more than expected in the third quarter.
The offshore Chinese yuan changed hands at 6.5478 per dollar , near its 2-1/2-year high touched last month, maintaining strength on expectations a Biden White House would be more conducive for China's economic growth.
The British pound held near a three-month high, last traded flat at $1.3387 after a choppy Wednesday trade as markets looked to whether Britain and the European Union can clinch a trade deal.
The negotiators may have enough progress to agree on a deal in the next few days, the BBC's political editor said, giving the pound a slight lift.
The EU's chief Brexit negotiator told member states' envoys on Wednesday negotiations were reaching "a make-or-break moment", and they urged him not to be rushed into an unsatisfactory agreement.
Four diplomats told Reuters after a briefing by Michel Barnier the talks remained snagged on fishing rights in British waters, ensuring fair competition guarantees and ways to solve future disputes.
As traders also brace for a make-or-break moment, the pound's implied volatilities have risen. Both the overnight and one-month volatilities have hit one-month highs, an indication investors expect choppy trade.
Bitcoin dipped 1.0% to $19,043, but stayed near its record high of $19,918 hit earlier this week. (Reporting by Hideyuki Sano; Editing by Lincoln Feast and Sam Holmes)