Flight Incheon International Airport departs from Seoul Incheon International Airport at 19:30 and arrives at Nha Trang Cam Ranh International Airport at 23:15.
The flighttime is about 5h 45m. The distance travelled ("as the bird flies"): 2289 Km. So the average speed is roughly 398 Km.
There is an 2 hour time difference between Seoul and Nha Trang