According to navy Adm. Ngo Van Phat, a regional commander, military radar recorded that the plane crashed into the sea south of Phu Quoc island. The People's Daily, Chinese Communist Party's official newspaper, reported the same information.
It says their radar confirms it fell into the sea. So, they must know where it fell in.
Looks like it crashed near where FlightRadar24 lost data.
Roughly 150 miles south of Tho Chu island.
https://pbs.twimg.com/media/BiLQ_9PCEAAS39o.png:large