Vietnamese language update

inc/lang/vi/backlinks.txt

inc/lang/vi/conflict.txt

inc/lang/vi/denied.txt

inc/lang/vi/edit.txt

inc/lang/vi/editrev.txt

inc/lang/vi/lang.php

inc/lang/vi/login.txt

inc/lang/vi/mailtext.txt

inc/lang/vi/newpage.txt

inc/lang/vi/norev.txt

inc/lang/vi/password.txt

inc/lang/vi/preview.txt

inc/lang/vi/read.txt

inc/lang/vi/revisions.txt

inc/lang/vi/searchpage.txt

lib/plugins/acl/lang/vi/help.txt

lib/plugins/acl/lang/vi/lang.php

lib/plugins/config/lang/vi/lang.php

lib/plugins/plugin/lang/vi/lang.php

lib/plugins/popularity/lang/vi/lang.php