Dusting is rather inconsistent in vampires, putting any sense to it is especially hard. Wood to the chest region does the job, however other then that, much sense can't be made from dusting.
Sometimes the object used to dust a vampire will turn to dust along with the vampire, when other times it stays. There really isn't much explanation for this. I'm going to have to say that there's a sort of magic aura around the vampire, a result of being a creature being preserved by magic. The aura makes things like clothes or things the vampire could be holding basically a part of them as far as the magic is concerned. If a thing is large enough then it is not part of them. Most of the time if another person is holding the object that does the dusting it doesn't disappear, but if stuck in the vampire deep enough it can.
On rare occasions things left behind in dusting could be because the object has some sort of spiritual bond with another person (For example a ring given in love might stay behind because it is spiritually attached to that person.)
The length it takes for a vampire to dust also varies a lot. Typically the more powerful a vampire is, or the amount of wood in them can effect the length it takes to dust, but this isn't always true. Overall, as far as dusting is concerned, it's all pretty random.