AWho is to say that they hadn't already tried and been unsuccessful at it? First of all, the vines were growing everywhere, including deep down underground in the shaft. Even if the surface vines could have been burned down, it's likely that the underground plants would simply resurface. Secondly, fire isn't the be-all/end-all solution for getting rid of unwanted plants. For example, bamboo can be burned right down to the ground year after year and still come back. Jack pine requires fire to open its cones so that the seeds can get out, so fire is actually beneficial for helping the plant to spread. Native prairies relied on occasional fires to sustain the native plants and destroy invading weeds and trees. In rural areas, controlled burning is often used in the fall to burn down standing weeds in ditches along the road or in fields so that uncontrolled fires can't spread through the countryside. The following spring, all those plants come back as usual. Some plants can even be more dangerous when burned. Poison oak releases the poison into the air, causing severe reactions and respiratory problems in people far downwind. It's even possible that burning the vines will cause them to spread over a much greater area and/or "take root" in the lungs when someone breathes the smoke. In short, burning the vines might not have stopped them, and the natives might already have figured this out.
ASalt inhibits plant growth. Salt is also used in magical practices to draw circles to either contain demons and evil spirits or to keep them away, because evil spirits (the vines?) supposedly can't cross over salt.
