Has it been verified that one of the last contacts signed off with the words "Roger that"? If so, then the investigators need to question as many crew as possible that flew with the Captain and FO to ascertain if they ever used those words. IF NOT, then I would investigate the individual who had flight sim training and see if he was taught or used these words in training.
Some have discussed in this thread -- and many more have discussed elsewhere, including in the lamestream media -- MAS370's last words to ATC. On one level, unless it was "Allahu akhbar!" or "Mayday, Mayday, Mayday!" who cares?
The two versions I've heard are either "Roger that," as quoted above, or "Alright, good night." Either phrase can be completely normal for a professional flight crew member to use, depending on what ATC said to engender the response.
Until we get a full/complete transcript of ATC comms, we don't know what was said by Malaysian ATC other than some version of "Contact Vietnam on such-and-such a frequency."