Its currently depending on virtual/x11. It would be a good idea to make vnc.so building selectable by use flag, and that would depend on xorg-server.
Aditionally, its installing its files under /usr/X11R6
In the meantime you can ln -s /usr/lib/modules/vnc.so /usr/lib/xorg/modules/vnc.so in case anybody is unaware ;)
I attached a new ebuild which should fix also this bug to bug 106724. Can you pelase confirm that it works for you?
I confirm it fixes this bug, but please see http://bugs.gentoo.org/show_bug.cgi?id=139244
thanks, I aded the ebuild from bug 139244 to portage.