Vala 0.15.1 (developer/testing release) is out. The ebuild from 0.14.1 renamed and with slot changed to 0.16 works for me on mostly ~amd64
Until either vala-0.16.0 is released or 0.15.x is needed for something in the main portage tree, slot 0.16 is purely overlay material. Will try to get 0.15.x into the overlay fairly soon, along with other gnome-3.3 packages.
*** This bug has been marked as a duplicate of bug 394107 ***