Why killed vampires are like ceramic which breaks?
They have super cold skin and they need fire to melt or kill them. It relates to how ice needs warmth to melt just like they need it to die. The shattering can relate to cold ice shattering just like glass just like their skin.
Their skin is super cold and they have to be burned with fire to die
extra stuff about when they are killed: they need fire to melt them because they are as cold as ice, ice needs warmth ( a fire ) to be burned and killed.
I am assuming that it it because given that their skin is decribed to be cold, and hard as glass, and that is sparles like a diamond, it is most appropriate the use the image of cracking when they are killed.