Prior to contact being lost, the flight had been airborne for just over 2 hours
Seems to be a lot of conflicting information, but there are now at least 4 sources (including the Vietnamese navy) putting it in the ocean between Malaysia and Vietnam, barely 40 minutes into its flight.
http://i.imgur.com/uoLG4bp.jpg
UPDATE [11:32am]: VN Express, Vietnam's largest news site, reports that Vietnam Emergency Rescue Center just announced it has found signal of the missing plane at 9.50am 120 miles South West of Ca Mau cape, the Southern-most point of Vietnam.
The signal is believed to be the ELT (Emergency Locator Transmittor) , which can be activated manually by the flight crew or automatically upon impact.
not looking good unfortunately...