A warning: even if you do get true genlock, you may still have a different kind of sync problem.
With vvvv, it appears the info is actually computed over the time of the frame readout from top to bottom, and things like a line moving sideways actually shows up on the monitor as a slanting line, what’s called a “rolling shutter” in cameras. When you do this over multiple stacked displays, you get a tearing effect, where the line at the bottom of the top monitor does not line up with the line at the top of the bottom monitor. It is quite obvious with fast moving graphics.
I have not found any Renderer (DX11) settings to force each frame render at a constant time; anyone know if there is such a thing?
Also, I would strongly suggest you use fewer PCs. I have done a 3x3 array of full HD displays with one PC, three outs from three graphics cards. Most cards now support up to four outs from each card, so you could do a 2x4 off one PC. It will make video playback much tighter and simpler (if that’s what you’re doing), particularly with DX11 - at least in my experience.