Why can't I access the SOAP or REST API when I use alternate vhosts?
Connect REST, Connect Web Services for SOAP, Knowledge Foundation API
For security reasons, SOAP and REST API access is only allowed on an interface's primary virtual host. Using an alternate virtual host will not work.
Information on WSDL retrieval can be found in the Introduction to Connect Web Services in the Getting Started section of the Connect Web Services for SOAP Developer Guide, found in the Answer ID 5169: Technical Documentation and Sample Code.