net-ftp/vsftpd-3.0.5-r1 installs /usr/share/empty/ instead of /usr/share/vsftpd/empty/. This is was fixed many rears ago for an earlier version, but the problem has reappeared. See bug #12834.
I think upstream uses /usr/share/empty. I don't see any reason to change it.
This is a change from where net-ftp/vsftpd-3.0.3-r4 put it.
The change is intentional. In older versions of vsftp, we alter the path via a source code patch (vsftpd-2.3.5-gentoo.patch). I see no reason to do so.
A Changes note to this effect would have saved me a few hours chasing a bug with a very vague error message.