On the left you can see Luke's face from the portion of the cover we're all familiar with. Below and to the right is our first glimpse of a Yuuzhan Vong, a (very) rough cropping of which I've made. I also brightened it a bit so the lit half of its face is somewhat easier to see.

I then examined the face as much as I could, and made a small sketch of what the face would look like in brighter conditions.

It appears very hard and skeletal. The YV may be decendants of insects, thus giving them a tough exoskeleton. The tentacles, on the other hand, are a completely different matter. I vaguely remember early reports of Jar Jar Binks as saying that several appendages resembling dreadlocks hung from the back of his head, and moved to indicate his emotions, much like a cat's tail. Maybe Salvatore caught wind of this concept and decided to use it with his characters. If so, the YV'S spead tentacles on the Vector Prime cover could indicate many different things, though I think anger or aggression is most likely.

UPDATE: I've replaced all my pictures of the dark Amazon.com cover with the B&N brighter one. We can now see the alien's face much more clearly, but figuring out what it is is still a challenge. JediNet's Ghentz, who is a strong believer in Chewbacca's death says that this may in fact be Chewie's decaying skull. Eew. While the face definitely seems to be a skull of some kind, I still think it's a Yuuzhan Vong. Call be stubborn, but I don't see how Del Rey would want the identity of the first major character to die in a book revealed simply by looking at the cover. I sure wouldn't.

UPDATE: Oh boy, was I wrong. =)