unless you're only ever going to be interested in particular lines (and no other) in the ftp server log, i'd shy away from processing it without exhaustively covering the full log format. the log format is covered in the documentation of the ftp server software. as this log format is configurable for the ftp server, the log output could suddenly change to another format. also i think this log will behave similarly to syslogd, in that if multiple same messages occur quickly, it will log something like "last message repated 500 times" in the log, rather than logging each one.
therefore it might be best to use a tool already written to parse vsftpd logs, rather than writing from scratch.