The default log location for vsftpd is not /var/log/vsftpd.log
It is on /var/log/xferlog
root@linux:~ # cat /etc/vsftpd/vsftpd.conf
# The target log file can be vsftpd_log_file or xferlog_file.
# This depends on setting xferlog_std_format parameter
xferlog_enable=YES# The name of log file when xferlog_enable=YES and xferlog_std_format=YES
# WARNING – changing this filename affects /etc/logrotate.d/vsftpd.log
xferlog_file=/var/log/xferlog
If syslog_enable is set, vsftpd will log everything to /var/log/messages