For more information about vsftpd, refer to the following resources.
The /usr/share/doc/vsftpd-<version-number>/ directory — Replace <version-number> with the installed version of the vsftpd package. This directory contains a README with basic information about the software. The TUNING file contains basic performance tuning tips and the SECURITY/ directory contains information about the security model employed by vsftpd.
vsftpd related man pages — There are a number of man pages for the daemon and configuration files. The following is a list of some of the more important man pages.
man vsftpd — Describes available command line options for vsftpd.
man vsftpd.conf — Contains a detailed list of options available within the configuration file for vsftpd.
man 5 hosts_access — Describes the format and options available within the TCP wrappers configuration files: hosts.allow and hosts.deny.
http://vsftpd.beasts.org/ — The vsftpd project page is a great place to locate the latest documentation and to contact the author of the software.
http://slacksite.com/other/ftp.html — This website provides a concise explanation of the differences between active and passive mode FTP.
http://war.jgaa.com/ftp/?cmd=rfc — A comprehensive list of Request for Comments (RFCs) related to the FTP protocol.