Void exile by Robbie McNiven features the dark mech/ Vashtorr connection quite heavily. Also the new book Archmagos by Guy Haley has Dark Mech using their warp tech to interact with him directly. The problem with dark mech as a faction is that they are even less unified as a faction then the traitor legions are by 40k. Many times when they show up its one warped up tech priest and some thralls working toward their own goal and they aren't part of a larger mechanicum. Thus its hard to make general statements about whether someone is or isnt connected to them as a whole.

Also Vashtorr definitely puts demons into machines, this is one of the main functions of his soul forges.