code: public Observable<ServiceResponse<List<VaultUsageInner>>> listByVaultsWithServiceResponseAsync(String resourceGroupName, String vaultName) {
          if (this.client.subscriptionId() == null) {
              throw new IllegalArgumentException("Parameter this.client.subscriptionId() is required and cannot be null.");
          }
          if (resourceGroupName == null) {
              throw new IllegalArgumentException("Parameter resourceGroupName is required and cannot be null.");
          }
          if (vaultName == null) {
              throw new IllegalArgumentException("Parameter vaultName is required and cannot be null.");
          }
          if (this.client.apiVersion() == null) {
              throw new IllegalArgumentException("Parameter this.client.apiVersion() is required and cannot be null.");
          }
          return service.listByVaults(this.client.subscriptionId(), resourceGroupName, vaultName, this.client.apiVersion(), this.client.acceptLanguage(), this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<List<VaultUsageInner>>>>() {
                  @Override
                  public Observable<ServiceResponse<List<VaultUsageInner>>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<PageImpl<VaultUsageInner>> result = listByVaultsDelegate(response);
                          List<VaultUsageInner> items = null;
                          if (result.body() != null) {
                              items = result.body().items();
                          }
                          ServiceResponse<List<VaultUsageInner>> clientResponse = new ServiceResponse<List<VaultUsageInner>>(items, result.response());
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }
docs: Fetches the usages of the vault.
  
  @param resourceGroupName The name of the resource group where the recovery services vault is present.
  @param vaultName The name of the recovery services vault.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the List&lt;VaultUsageInner&gt; object