Any Camera that has a live-view output to HDMI can be used, e.g. the 5D Mk III, which recently received full 4:2:2 output capabilities in a firmware update. Just make sure that you can disable status display on the external output.

I'm sure there are some that would be willing to give you step by step instructions on how to connect a 5D to vMix, but I think that kind of hand holding probably would probably require some sort of incentive.

Canon 5DmII and 5DmIII both have mini HDMI outputs. So you'll be able to go directly into any of the supported HDMI capture devices.

(you can find a list of supported capture devices here http://www.vmixhd.com/supported-hardware.aspx)

A couple of things that you're going to want to be careful about:

5D sensors are not designed to run continuously for hours. They have a tendency to overheat.
ex: http://www.dpreview.com/forums/post/33102644
ex: http://photography-on-th...showthread.php?t=1230974

5DmII and 5DmIII do not support auto focus during live view. If you're capturing an event, you'll need to be pulling focus the whole time. The problem with this, is that your focus is on the lens, and any time you touch the lens, it's going to impact the stability of your image. Live streaming an unstable image is not suggested... for the viewer's sake.

5DmII and 5DmIII do NOT work well when shooting in low light environments.

If you need more support than that, you might want to contact someone on the forums directly about a consulting contract.

just my $.02