This is much easier now just use https://learn.microsoft.com/en-us/dotnet/api/azure.security.keyvault.certificates.certificatemodelfactory.keyvaultcertificatewithpolicy?view=azure-dotnet

Example:

var fakeCert = CertificateModelFactory
  .KeyVaultCertificateWithPolicy(certificateProperties, cer: new byte[] {1,2,3});