"He <Vain> smiled ambiguously as one of the stars struck and burst against his right forearm. Another concussion shocked the caver. Ebony fire spat like excruciation from the Demondim-spawn's flesh. When it ended, his forearm had been changed. From elbow to wrist, the skin and muscle and bone were gone, transformed into rough-barked wood. Deprived of every nerve or ligature, his hand dangled useless from his iron bound <heal from staff of law> wrist."
I apologize if this has been previously discussed; a very quick search did not reveal the topic. But it just hit me (yes, I can be very slow) that this needed to happen to Vain, and Seadreamer knew it. The stars were a defense for the One Tree, provided by the Worm of the Wolds End. My thinking is that part of Vain's creation was to be able to absorb the touch of the stars, and convert that power into wood exactly like the wood from the One Tree - if not actual wood from the one tree.
SPOILER IF YOU HAVE NOT READ "WHITE GOLD WIELDER"