The log file is /var/log/vsftpd.log
You could use grep, awk, or sed to do what you need to. Are you talking about all users, or just a specific user? If it is a specific user, you can just use grep, like so:
Code:
cat /var/log/vsftpd.log | grep user
I don't use vsftp, so that is a start. You could also use sort if need be. Hope that helps,
Josh