Flight Incheon International Airport departs from Seoul Incheon International Airport at 04:00 and arrives at Ho Chi Minh City Tan Son Nhat International Airport at 05:45.
The flighttime is about 3h 45m. The distance travelled ("as the bird flies"): 2519 Km. So the average speed is roughly 672 Km.
There is an 2 hour time difference between Seoul and Ho Chi Minh City