vsftpd usually logs to it's own configured log in /var/log like /var/log/vsftpd.log. Check your configuration, if it's not configured, it's probably by default logging to messages or syslog, in which you may want to change. I'm not sure of any specific commands for vsftpd but you could probably write a script to parse the logs to find out who's logged in and at what times, etc.
Edit: Didn't realize you only said sftpd and not vsftp. What ftpd daemon are you specifically running?
Last edited by trickykid; 08-18-2008 at 10:24 PM.