Does one have to fly over Vietnam from China to Thailand?

It’s hard to say. For example, if you take off from airports in eastern and western China, you may not necessarily pass through Vietnam. But the problem is that although the flight route is either a straight line or an arc, it may still be halfway through depending on the situation. You may have to take more passengers with you when you stop to refuel, because this is determined by the air traffic control in different regions and the airline's own choice and arrangement of flight routes. It does not mean that as long as you fly from China to Vietnam, you will definitely pass through Thailand. Part of the time is not as simple as a straight line or an arc! Unless you are given a plane and you risk invading other countries' airspace and being shot down and trying to fly to Thailand via Vietnam, it would be more reliable to inquire more about the status of each flight!