Configuring Vagrant CA Certificates
You don't explain what steps you have taken to try to fix the issue, but it would appear that you are not placing your root certificates in the correct location.
Go to the directory where you installed Vagrant, find the file embedded\cacert.pem, and then append the contents of your corporate certificates to the file. Save it and retry the operation. If you properly exported your root CA certificates then they should be read by Vagrant and allow the connection.
If you are still unable to make it work by combining the files, try running vagrant with SSL_CERT_FILE=/path/to/your/certs.pem in the environment. This will allow you to validate that you have properly exported your corporate certificates.