My big problem with it is why did the vampires clothes turn to dust? And even then it’s pretty inconsistent…that one time Angel staked a vampire from Las Vegas, his sunglasses didn’t turn to dust with the rest of his outfit, so Angel took them. 
It’s magic!
Fanwank: The clothes turn to dust if they are what the vamp was wearing when they are turned. We know they tend to keep the same clothes - Buffy commented on the bad fashions in the pilot. Have we ever seen a vamps clothes dust that were obviously not what they were wearing when turned? That would explain the sunglasses, at least.
Any vamp more than 50 years old or so is generally seen wearing modern clothes. For example, Darla is dusted with clothing.
You forgot to wave your hands!
Simple. The sunglasses were made of phlebotenum.
Or maybe plotonium. Oh, man! Plot - onium! Substance which moves the plot! It’s a *way *better word than phlebotenum. Did I just invent it? Pretty please?
drat.
I believe Angel was just very quick on the draw, and grabbed them before the dusting spread that far.
In “Doomed”, Xander was concerned about staking Spike while he’s wearing one of Xander’s shirts, because he didn’t want it dusted.
Darla was definitely wearing different clothes than from when she was vamped, but her clothes still dusted with her. Both times.
*** Ponder
Then obviously it’s the animating power of the demon that causes everything to dust, including the clothing. This might also explain why newly-created vamps dust almost instantly when staked, while older, more powerful vamps can often get a farewell line in before disintegrating (see Mr. Trick).