Question: When Bond realises he has been poisoned at the poker game, he mixes salt with water and drinks it. What was this supposed to do? Does salt mixed with water rid the body of hazardous chemicals or something?
Answer: Thiopentone Sodium, Sodium Nitrite, and Sodium Thiosulfate are the most common chemicals used to treat cyanide poisoning. Cyanide might've been Bond's first thought, and he was ingesting large quantities of sodium to try to stave off the effects.
Question: One thing I didn't understand. If Chiffre's organisation had kidnapped Vesper's boyfriend already in return for the money, how could Vesper have struck a deal for them to spare Bond in return for the money? Couldn't they not accept since they already have the upper hand (Vesper's boyfriend)?
Answer: Vesper's boyfriend was a member of Quantum himself. He wasn't actually in danger, but his kidnapping was staged to gain leverage over Vesper. Since they were getting the money anyway letting Bond live didn't seem much of a stretch, he was only a raw agent at the time.
Chosen answer: He did it to simply make himself vomit, which is what he does into the sink. Inducing vomiting is meant to rid the stomach of the poison that hasn't been absorbed (although this idea is debatable, Bond thought it was wise). However, he was still in danger because he already absorbed enough of the poison, which is why he goes to the car and injects the antidote (the injection he gives himself in the neck).
Bishop73