The plugin depends on network library yavijava
https://github.com/jenkinsci/vsphere-cloud-plugin/blob/master/pom.xml#L95-L96 

The yavijava supports two types of http clients:
CloseableHttpClient
HttpURLConnection - is default
The HttpURLConnection client's behaviour can be impacted by system-wide cookie handler CookieManager:
https://docs.oracle.com/javase/8/docs/api/java/net/CookieManager.html The major problem is that the CookieManager overlaps logic provided by [WSClient] to setup http header Cookie. Global CookieManager know nothing about logic of vsphere-cloud-plugin and fill all http headers with stored cookies, as result vsphere center rejects login requests with error:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <soapenv:Body>
        <soapenv:Fault>
            <faultcode>ServerFaultCode</faultcode>
            <faultstring>Cannot complete login due to an incorrect user name or password.</faultstring>
            <detail>
                <InvalidLoginFault xmlns="urn:vim25" xsi:type="InvalidLogin">
                    <faultMessage>
                        <key>vim.fault.InvalidLoginWithReason</key>
                        <arg>
                            <key>1</key>
                            <value xsi:type="xsd:string">vim.fault.InvalidLoginReasonRegisterFailed</value>
                        </arg>
                        <message>Login failed because the session could not be re-registered.</message>
                    </faultMessage>
                </InvalidLoginFault>
            </detail>
        </soapenv:Fault>
    </soapenv:Body>
</soapenv:Envelope>

The vsphere-cloud-plugin does not use CookieManager, but there is no guarantee that other plugins avoid it. Example:
https://github.com/jenkinsci/parameterized-remote-trigger-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ParameterizedRemoteTrigger/RemoteBuildConfiguration.java#L158-L159 

As workaroud we can switch to CloseableHttpClient http client.

I have prepared draft pull request:

https://github.com/jenkinsci/vsphere-cloud-plugin/pull/133

Please also pay attention to the issue in repository yavijava:

https://github.com/yavijava/yavijava/issues/290