pam_pwdfile is hard-masked now and supposed to be removed from the portage tree soon. It might be unlucky, because vftpd uses pam_pwdfile for authentification of virtual users. Usage of pam_pwdfile for this purpose is widely described (for ex. http://en.gentoo-wiki.com/wiki/Vsftpd), without this module there is only one documented method (pam_userdb) left. But pam_userdb is already considered obsolete, and already absent in the portage tree. Reproducible: Always Please, review the decision about removal pam_pwdfile from portage, otherwise there would be no described method, how to set virtual users for vsftpd, and many documents about vsftpd will be confusing or partly invalid.
Sorry but exactly like the webmin case ( http://blog.flameeyes.eu/2010/08/20/there-s-something-about-webmin ) if there is nobody maintaining the package we're not going to just keep it around; it's also missing upstream. As for pam_userdb, it's present as part of sys-libs/pam when you enable berkdb USE flag, the separate ebuild has been removed, because it is now part of the main one. So this is WONTFIX unless you decide to pick it up, both upstream and in Gentoo, or you hire somebody to.
