Ha I actually read about this in one of those Cannabis Culture magazines, debating whether lighters or matches are better for a cleaner smoke. It said the cleanest smoke can be gotten by using a magnifying glass on a sunny day, and I definitely see that as true. You are using only the sun's energy to light it, nothing else. I don't think it would vaporize it depending on how strong you concentrated the sun's rays, if you got it down to one tiny point it would probably be hot enough to burn it. This would be fun to do for novelty purposes, and srw96 put it to practical application, but for normal use it would probably be an inconvenience.