Since incineration involves a chemical change, it’s not typically a reversible process. From a vs standpoint, this means if something’s entirely incinerated, it’s more difficult to regenerate than if it’s vaporized. It also is why burns are often depicted as stopping regeneration in fiction.
From a calc standpoint, combusting something to incinerate it will pretty much always require less energy than vaporization. If something is combustible, then that means it chemically decomposes before it’s molecules can become excited enough to become gases, so the heat and energy required has to be lower. Additionally, when something combusts, the breakdown can create additional heat, so not all the energy has to come from the character that started it (for instance, when you light a match, the energy of the flame is significantly more than the friction you used to ignite it).