VC:SdkConnection has a property instanceUuid, which is kind of UUID of the corresponding vCenter server.
VAPI:VAPIEndpoint has no such property. It is a simply alias for vAPI endpoint URL + username/password. So you'd need to figure out if this vAPI endpoint is a vCenter endpoint (it could be something else; eg. NSX endpoint). Then, you'd need to figure out if there is a corresponding vAPI service/method that returns vCenter's instanceUuid eg. as part of some 'about' info. I don't think current vCenter vAPI exposes such service but I might be wrong here.