After setting up VSFTP i wanted to support FXP. It took some tweaking to get this working and eventually these settings seemed to work. (Admins - i guess this might be better off in the VSFTPD page rather than here - please feel free to move it).
By default most ftp connections use passive mode - which means the server needs not only port 21 open, but a group of ports (usually over 1024) open for data connections... i usually keep about 20 ports open for an ftp expecting 2 simultaneous downloaders...
Anyways, you can either change your ftp client so it connects to the server in ACTIVE mode - but you'll have to make sure everyone does that.....or else you'll configure your server and vsftpd for passive connections, open a set of ports on your firewalls say from 2001 to 2021
(Obviously port forward these on your firewall to your Slug IP)
Add this to your /etc/vsftpd/vsftpd.conf
You also need to add this if you're planning to use FXP with this server (server to server transfer)
I hope that helps somebody - it has worked for me. If this is unsecure at all please recommend any changes.