Flight Incheon International Airport departs from Seoul Incheon International Airport at 09:00 and arrives at Ho Chi Minh City Tan Son Nhat International Airport at 14:35.
The flighttime is about 7h 35m. The distance travelled ("as the bird flies"): 2519 Km. So the average speed is roughly 332 Km.
There is an 2 hour time difference between Seoul and Ho Chi Minh City