The appstream ebuild in gentoo is missing vala support. It would be great if this could be incorporated. Currently I have it enabled on an overlay, but given the minor changes (adding vala USE flag, vala deps, and update vala configuration flag) I'd say this should be possible to include in the gentoo tree as well. See https://github.com/pimvullers/elementary/tree/master/dev-libs/appstream for updated ebuilds with vala support.
*** This bug has been marked as a duplicate of bug 733768 ***