You can update that field by directly interacting with the IaaS API's.
//Update the item with the new description field
var modelName = 'ManagementModelEntities.svc';
var entitySetName = 'VirtualMachines';
var entityIdString = vCACEntity.keyString;
var links = null;
var headers = null;
var updateProperties = new Properties();
updateProperties.put("Notes", "Your new description field here");
System.getModule("com.vmware.library.vcac").updateVCACEntity(vCACHost.id, modelName, entitySetName, entityIdString, updateProperties, links, headers);