Hanoi to Istanbul is about 7,417 km and usually takes around 10h in the air. The track it follows runs over the Caucasus, the Anatolian plateau, and the Black Sea.
Air lifting over the Caucasus and the Anatolian plateau 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.