Flight Incheon International Airport departs from Seoul Incheon International Airport at 06:50 and arrives at Nha Trang Cam Ranh International Airport at 09:50.
The flighttime is about 5h 00m. The distance travelled ("as the bird flies"): 2289 Km. So the average speed is roughly 458 Km.
There is an 2 hour time difference between Seoul and Nha Trang