In order to backup your cPanel VPS to your remote FTP backup account, you need to login to your WHM. Then click on Backup > Configure backup.
The options are very straight forward, and you simply need to choose how frequently you want to backup your VPS, and what to include. Then, lastly, choose the remote FTP options, supply your username & password, and click on save.
For further info, click on the following link: http://www.cpanel.net/support/docs/11//whm/backup.html