I have an FTPS server running VSFTP and below is collection of commands useful for monitoring activity and analyzing the logs.

Watching user connection processes:

Watching user network connections:

Show process name, PID, and file descriptor for each open FTP connection:

Show output of ps -ef for each user connection process:

Checking per-user bandwidth utilization:

Get VSFTP server login and transfer statistics:

Sample output:

For more VSFTP log analysis examples, read Log Event Time Distribution