Siliconera notes, the independent developer has said it's looking down many development avenues, so it's not clear what Vidrhythm may be. Go ahead and place your bets.
Officially, Harmonix is currently working to high-kick Dance Central 2 out the door and continues to pump out Rock Band DLC at a weekly beat. We've contacted the developer for any further details on Vidrhythm.
Update: Harmonix communications director John Drake informed Joystiq: "Nothing to add at this time, other than to confirm that it is a project we're working on."