Istanbul to Hanoi is about 7,417 km and usually takes around 10h in the air. The track it follows runs over the Anatolian plateau, the Black Sea, and the Caucasus.
Air lifting over the Anatolian plateau and the Caucasus can raise brief, light ripples at cruise; crews know these spots well and the movement stays gentle. Over the Black Sea the air is usually calm and settled. This track threads jet-stream latitudes, so in winter you may feel a few minutes of light, choppy air high up; summer usually rides calmer.
A flight this long spends hours at a high, smooth cruise. Even when one stretch is bumpy it is a small slice of the whole trip, and the aircraft treats it as routine.