Flight Incheon International Airport departs from Seoul Incheon International Airport at 09:15 and arrives at Hanoi Noi Bai International Airport at 12:05.
The flighttime is about 4h 50m. The distance travelled ("as the bird flies"): 2409 Km. So the average speed is roughly 498 Km.
There is an 2 hour time difference between Seoul and Hanoi