I have having the same problem, but with Deus Ex Human Revolution: Directors’ Cut. It will start just fine if VorpX is disabled. When VorpX is running, it tries to start and fails with the “Failed to start game (unknown error)” Steam error. Other games (e.g Skyrim, Bordlands 2) work just fine. Metro 2033 Redux and Serious Sam also worked fine (after I did the VorpX Desktop trick once). DEHR:DC, however, still fails when VorpX is running, even using VorpX Desktop.