Are you wanting to show something like the aftermath of those recent California wildfires? Since you said vignette, I'm guessing the size of your scene will be on the smallish side. So what Tim mentioned would work, as would chalk or pigments. If you have any structures, like a wall or building corner, etc., the same materials & methods should work. Now comes the risky(crazy??) idea. A few years back, I made a small dio in which I wanted to have a partially burned out barn/shed where the roof had sustained the most damage. Using wall parts from an old ceramic kit, I attached balsa wood beams & scratch-built shingles. Then using one of those candle/fireplace butane lighters, I carefully "scorched" portions of the beams/shingles till I got the look I wanted. I've used small pieces of burned balsa or basswood at other times as well. In some instances, actual burned wood adds more realism, at least to my eyes.