I have seen that fedora is simply specifying 0.24 over our used 0.20, I think would be interesting to try to use newer one (as 0.20 is not really maintained by upstream for a long time and we still have it as a few packages are still not ported to newer vala versions) Thanks
+*xfce4-vala-4.10.3-r1 (14 Jan 2015) + + 14 Jan 2015; Pacho Ramos <pacho@gentoo.org> +xfce4-vala-4.10.3-r1.ebuild: + Work with newer vala version (#527864) +