All cPanel servers come with self-signed certificates configured to secure the control panel as a default feature. Self-signed SSL certificates are the cause of this warning. Since SSL certificates are only valid for the domain(s) they are purchased for, the majority of our reseller clients that wish to offer cPanel control panel access to their customers without the certificate warning purchase a signed certificate from a major authority (GeoTrust, Comodo, etc.) for the hostname of their VPS server. (ServInt customers can purchase trusted certificates and have them installed from ServInt directly filling out a ticket in the customer portal.)
Once a trusted certificate is installed, cPanel can be configured to redirect all requests to the SSL port, and redirect all access to the hostname of the server. Therefore, if you purchase a certificate for the hostname of your VPS server (typically vps.xxxx.com), the control panel can be configured to route all requests to this domain, and thus secure all access to the control panel without the SSL warning.
Photo by Yuri Samoilov