Certificates for the virtual appliances must be in PEM format.
If your certificate authority provided a certificate in PFX format, use OpenSSL to convert PFX to PEM.
openssl pkcs12 -in path-to-pfx -out desired-path-to-pem -nodes
openssl pkcs12 -in C:\vra-cert.pfx -out C:\vra-cert.pem -nodes
You might be prompted to enter a pass phrase if the PFX certificate included one.