There is no ignition power in the Vec C's HU loom only a constant power and Can/trigger wire. To get your aftermarket HU to work you need to take an ignition live feed from the passenger door fuse box or from the ciggi lighter mate. Vec C dont have the old system of a constant live and Ignition live, only a constant and a trigger. You could get a Can bus interface but its not neaded, just take the ignition live as said. I believe the proper VX single din adapter comes with the wire to do this but you can just splice into it with your own wire.
If you need any more info just ask, Ive got a DVD player in the glovebox and screen in the roof so I know all the best places to take feeds from.