Monitor network traffic for several users

Hi

Im setting up a debian box, rented from leaseweb.
Two people are gonna use it as seedbox.

So far I've installed vsftpd, xterm, fluxbox, vncserver, vnstat and wine, to use with utorrent.
Ive set up two users on the box.
I want something to monitor the bandwidth of the box, how much each of the two users use with utorrent and ftp-transfer.

Is this possible?
Havent managed to do this with vnstat.

Thanks

if you are new to linux ...
the simpliest way to monitor "everything" is

sudo apt-get install webmin

then connect from any pc in your network via:
https://IP.IP.IP.IP:10000

user:root
pw:????

;)

(if you search you will find the button called MONITOR NETWORK TRAFFIC) ^^