If Vesper Lynd was working with Le Chiffre in Casino Royale, why did he have to force Bond for the password when she could have stolen the money and given it to Le Chiffre anyway?
Vesper is not working with Le Chiffre, though they are working for the same employer. Le Chiffre is in deep with Quantum following the loss of the warlord's money and needs the winnings to cover his debts. Vesper is there to sabotage Bond's chances, because Quantum has leverage over her. She has no loyalty to Le Chiffre, as seen by the second deal she cuts, to save Bond while he is being tortured. Mr. White of Quantum intervenes and eliminates Le Chiffre because the money is Quantum's goal, regardless of how they get it. Le Chiffre is expendable to them.
It's also important to note that Vesper does not have the password at the time of the torture, Bond reveals the password (her name) later, while he is recovering.