The following should return the UUID of your vCenter Server assuming the input object is a vCloud:ProviderVdc object named "providerVdc"
var vcdHost = providerVdc.getHost();
var pool = providerVdc.toAdminExtensionObject().getResourcePools()[0]; // get the first resource pool (VclVMWProviderVdcResourcePool)
var vimObjectRef = pool.resourcePoolVimObjectRef;
var vimServer = vcdHost.getEntityByReference(VclFinderType.VIM_SERVER,vimObjectRef.vimServerRef);
System.log("VimServer uuid: "+vimServer.uuid);