media-sound/vorbis-tools-1.1.1-r1 (and possibly 1.1.0 too) needs >=media-libs/libvorbis-1.1.0; otherwise (if using libvorbis-1.0) compilation fails. In the ebuild >=media-libs/libvorbis-1.0 is given; admittedly, configure script in the package itself also only checks for >=1.0, but the source code assumes that version 1.1 is available. (I'll report this upstream for the configure fix, but the dependency in the ebuild should be fixed nonetheless.)
Update: no need to report upstream, it's already fixed in the repository: http://trac.xiph.org/cgi-bin/trac.cgi/changeset/9622
This has been updated in CVS; thank you for your report.