Sir Miles wrote:...they knew that Vesper would just tell them the bank details, they needed Bond for the password....and it's much easier for them to get it through Vesper than through torture...
You say the threat of violence against Vesper wasn't real....why not ? It was far easier all round to get Vesper to get the information from Bond [exactly how she did] rather than waste time torturing him...that doesn't make sense...
The torture scene only makes sense [to me at least] IF Le Chiffre doesn't work directly for Quantum and therefore can't get his money before Quantum come for him...otherwise Le Chiffre would just ask Vesper to ask Bond for the password and he'd have his money in a matter of minutes...
As for Mathis..?..there is very little proof that he's not a double-agent...apart from the line in QoS....I guess you pays your money and takes your choice... 
I'll try to explain my thinking on this. I didn't think that Vesper had direct control over the money. Even though she says "I'm the money" once Bond gets the money, it's his. Remember the banker. Now, when Bond needs more money, Vesper probably would need to put in a request. Which she didn't want to do. She wanted Bond out of the game to save her "boyfriend" who she thought would be killed. Bond didn't want to ask M for the money because Vesper would probably complain that Bond is too reckless and would just lose the money again.
I always assumed Vesper told Le Chiffre (either directly or indirectly) that Bond knew his tell. As I mentioned I think in this or another post, Eva Green gives a great performance as Vesper through her facial expressions. For example, after Bond gets back in the poker game, Vesper is clearly angry.
On the torture, I vaguely remember in the book that Vesper wasn't tortured. In the movie, Bond was going to take the password to his death. Le Chiffre enjoyed torturing him for putting him through all the trouble in the poker game. Le Chiffre probably even assumed that Bond is the guy who caused all his plans to go wrong. Anyway, after Bond wins the poker game, Vesper had no real reason to ask for the password. If anything, Vesper asking for the password would have made Bond suspicious. Torture with the threat of violence against Vesper was the best route at that time. Both for Le Chiffre who desperately needed the money and Quantum since they were most likely unsure if Vesper could get it back. Remember, Vesper didn't give Bond the extra money. She didn't know if she could get the password. So, if Bond sees Vesper injured he would correctly assume that they are going to kill them anyway. The scream is the threat of harm. If Vesper was unharmed, Bond would think they aren't serious enough and there is the possibility of escape. Then Vesper could get the password, which is pretty much what happened.
"Better late than never."