I go the following error when trying to launch Vault Client, and errors about being unable to check out a file in Visual Studio. I discovered that this was because I had the Fiddler tool running. What Fiddler does is edit your machine's Internet Explorer Internet Settings so that Fiddler acts as an HTTP proxy so that it can trace all HTTP responses and requests.
This can be fixed by temporarily shutting down fiddler when not needed. It may also be possible to configure Vault Client to not use a proxy, but I'm not sure if this will be possible in Visual Studio.Unable to connect to http://servername/VaultService. No server was found at the specified URL. Please verify your network settings using the Options dialog under the Tools menu in the Vault GUI CLient. Web Exception: The request failed with HTTP status 403: Access Forbidden.