code: private Datastream getDatastream(FedoraClient fedora, String vitalPid,
              String dsPid) {
          try {
              return fedora.getAPIM().getDatastream(vitalPid, dsPid, null);
          } catch (Exception ex) {
              log.error("API Query error: ", ex);
              return null;
          }
      }
docs: Get the indicated datastream from VITAL. This method pre-supposes that
  the datastream does in fact exist. Call datastreamExists() first to
  confirm.
  
  @param fedora An instantiated fedora client
  @param vitalPid The VITAL PID to use
  @param dsPid The datastream ID on the object
  @returns Datastream The datastream requested, null if not found