Waypoint change in 1:07 ACARS message?
If you wanted to "steal" an aircraft (and I don't) wouldn't you want it to look like it had simply disappeared? Presumably they'd have been flying on LNAV (maybe VNAV too). Since there is circa 10 miles IIRC without radar cover between Malaysia and Vietnam what was to stop whoever was flying choose heading select towards IGARI, enter VAMPI as next waypoint in the FMC, turn further 40° right after IGARI still on heading select then go back to LNAV? Now you see us, now you don't!
777 drivers, does the content of outgoing ACARS messages show on EICAS or MCDU? If it does, maybe it was noticed, hence the turning off of ACARS. Transponder off would have been a given, to appear "invisible".
Lets just hope MH370 is found soon.