Flight Incheon International Airport departs from Seoul Incheon International Airport at 20:30 and arrives at Nha Trang Cam Ranh International Airport at 23:45.
The flighttime is about 5h 15m. The distance travelled ("as the bird flies"): 2289 Km. So the average speed is roughly 436 Km.
There is an 2 hour time difference between Seoul and Nha Trang