the ebuild should either install a PAM config file for vsftpd or provide instructions for the user to perform installation manually.
Erm, vsftpd depends on net-ftp/ftpbase which installs the needed files for all ftp daemons that support ftpbase.
except, apparently, /etc/pam.d/vsftpd which is required by vsftpd when using PAM. file a new bug report, or reopen this one? net-ftp/ftpbase-0.00 got installed as a dependency of net-ftp/vsftpd-2.0.3-r1
No, there's no bug here at all. The settings are done in /etc/pam.d/ftp, for every ftp daemon in Gentoo.