Disappear without trace (6)
'disappear without trace' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vanish that I've seen before include "Go missing" , "Become invisible" , "Cease to exist" , "Disappear (in a puff of smoke?)" , "Disappear without trace" .)
(Other definitions for evaporate that I've seen before include "Fade away gradually" , "Change from liquid to gas" , "Change from liquid to vapour" , "Disappear into thin air" , "Dissolve into the air" .)