vsftpd.conf
...
setproctitle_enable=YES
...
If enabled, vsftpd will try and show session status information in the system process listing. In other words, the reported name of the process will change to reflect what a vsftpd session is doing (idle, downloading etc). You probably want to leave this off for security purposes.
Now you can do a ps axuw | grep vsftp (or whatever you like) and see connected users.
[root@RHEL56 ~]# ps axuw | grep vsftpd |grep -E :\ connected'
nobody 2141 0.0 0.0 46384 1140 ? Ss 11:46 0:00 vsftpd: 192.168.1.56: connected
nobody 2341 0.0 0.0 46384 1140 ? Ss 11:45 0:00 vsftpd: 192.168.1.96: connected
nobody 2541 0.0 0.0 46384 1140 ? Ss 11:48 0:00 vsftpd: 192.168.1.46: connected
Done