I'm pretty sure she didn't. The two of them were together from the moment they came into the restaurant, when would she have had a chance to?
When Vaughn asked her "Did you ask him to do that", he was sort of kidding, you know, being flirtatious. She was embarrassed and sort of amused, that's what that smile was, I think. They were both surprised by being offered the key. After all, Vaughn asked her out. I don't think she would all of a sudden decide once they got to the restaraunt that this would be a good time to sleep together. That's out of character for Sydney.
When Vaughn asked her "Did you ask him to do that", he was sort of kidding, you know, being flirtatious. She was embarrassed and sort of amused, that's what that smile was, I think. They were both surprised by being offered the key. After all, Vaughn asked her out. I don't think she would all of a sudden decide once they got to the restaraunt that this would be a good time to sleep together. That's out of character for Sydney.