I tried to add mask value with 'volumeStatus', but it seems that it allways returns null.
Service service = Evault.service(client, Long.parseLong(storageId));
service.withMask().volumeStatus();
This request can help you:
https://[username]:[apikey]@api.softlayer.com/rest/v3/SoftLayer_Account/getEvaultNetworkStorage?objectMask=mask(SoftLayer_Network_Storage_Backup_Evault_Version6)[agentStatuses]
Method: GET
References: