Roland Edirol V-8 is a basic VJ video mixer, in a standard DJ mixer format and does the job for mixing between 8 video sources.
The V-8 doesn’t have a Tally light interface, but it does have MIDI out.
Using an Arduino, Sparkfun MIDI Shield I already had, and Tim Twillman’s Arduino MIDI library, it was fairly straight forward to check the MIDI Control Change messages.
Using a series of IF statements, that would keep an eye on which input buttons had been pushed and was position the Transition bar was in (all the way left/centre/all way right).
Sparkfun MIDI Shield: http://www.sparkfun.com/products/9595
Custom PCBs: itead PCB Prototyping Service