Flight Incheon International Airport departs from Seoul Incheon International Airport at 09:10 and arrives at Ho Chi Minh City Tan Son Nhat International Airport at 12:50.
The flighttime is about 5h 40m. The distance travelled ("as the bird flies"): 2519 Km. So the average speed is roughly 445 Km.
There is an 2 hour time difference between Seoul and Ho Chi Minh City