Certificate is in another format and VDS requires that it be converted to PEM.
Here is a list of steps to follow to convert your certficate from PFX to PEM.
This can be done on either Windows or Linux, as long as you have openssl installed.
Take your PFX certificate and convert it using this command:
openssl pkcs12 -in mycert.pfx -out mycert.pem -nodes
Enter Import Password:
MAC verified OK
Then open the .pem file in Wordpad (not Notepad)
Remove any extra text that there may be, outside of the <KEY> tags and save the changes.
Enter the path to the new certificate file in your VDS config and click the blue "i" icon to confirm that it displays the certificate details. If this box is empty, there is a problem with your certificate.