To restrict an FTP user to specific domains on a VPS first that FTP user must be the administrative user for the account. You can set the administrative user for a site in your HSPC control panel with the following steps:

1. Click on the Site tab.
2. Click on Select Another Site.
3. Select the site for which you wish to change the administrative user.
4. Click on Website Settings.
5. Click on the Configure button.
6. In the Administrative User section, select "Existing user".
7. From the drop-down list, select the user you wish to be the administrative user.
8. Click on the Update button to save your changes.

This will place the site files for this site to:

/home/username/domainname.com

Replace "username" with the FTP username and "domainname.com" with the actual domain name.

Next you need to set up a default chroot for your user for proftpd. You will need to edit your /etc/proftpd.conf configuration file and add a line towards the end of the configuration file similar to:

DefaultRoot /home/username username

Change "username" in both places to the actual FTP username.

For more information on the use of the DefaultRoot directive see the following link at the proftpd.org web site:

http://www.proftpd.org/docs/directives/linked/config_ref_DefaultRoot.html

For the changes in your proftpd.conf to take place you will need to restart your proftpd service. If proftpd is running through the xinetd daemon you will need to restart the xinetd service instead.