[SalesForce] Web service callout failed: WebService returned a SOAP Fault: System.StringException: Invalid id

It's possible to scaffold a System.CalloutException by deserializing it into existence or instantiating it with the type. Then you can throw it in your mock:

Adrian Larson notes that as of API v36.0 you can just construct them :-)

@TestVisible class ResponseMock implements WebServiceMock {
    public void doInvoke(
        Object stub,
        Object request,
        Map<String, Object> response,
        String endpoint,
        String soapAction,
        String requestName,
        String responseNs,
        String responseName,
        String responseType
    ) {
        CalloutException e = new CalloutException();
        e.setMessage('Validation Error(s) occurred during Foo Get.');
        throw e;
    }
}

Inject it into your test with the normal HTTP mock method:

Test.setMock(WebServiceMock.class, new ResponseMock());
client.doTheCallout();

pluto,

just like you take the sessionId from loginResult and set it to sessionHeader, you need to take the serverURL from loginResult and assign it to the api object endpoint URL.. and the sessionHeader you initialized is not being assigned to the actual callout object.

vNapp.SessionHeader = ses;
vNapp.URL = lr.serverURL;