Flight Incheon International Airport departs from Seoul Incheon International Airport at 10:40 and arrives at Vientiane Wattay International Airport at 13:50.
The flighttime is about 5h 10m. The distance travelled ("as the bird flies"): 2770 Km. So the average speed is roughly 536 Km.
There is an 2 hour time difference between Seoul and Vientiane