vSphere Client is no longer available starting with vSphere 6.5, but here’s a link to all the older versions:
https://kb.vmware.com/selfservice/microsites/microsite.do?cmd=displayKC&docType=kc&externalId=2089791&sliceId=1&docTypeID=DT_KB_1_1
Now the HTML 5 client is embedded in both the vSphere and vCenter appliance. This allows for administration directly if certain services fail. You can access the either by https://<ip>/ui or https://<FQDN>/ui.