I had the same error message running SAGA vector algorithms. You may have tried this, however, to get the algorithms to run (using QGIS Dufour and SAGA 2.1 - ships with the QGIS 2.0.1 windows standalone installer), I needed to ensure that the option to "Enable SAGA 2.0.8 compatibility" within the processing configuration and options settings was turned off (unchecked). I also had to alter the folder path to where the saga.gui was located following the standalone installation of GIS. I then restarted QGIS, and the algorithms worked.
Hope it assists.