I would go affliction on your lock.

You can still use your voidwalker to tank. put 5 points in improved corruption to get a second instant dot.

Send in voidy - give it a second to establish aggro, then double dot the target (throw on an immolate if you fancy it too), Either wand or when you have lifetap (make sure you improve it with talents) then life tap a couple of times, and drain life the mana back. When high enough, get nightfall for instant shadowbolt extra damage.

I think at higher levels a shadow priest and lock complement each other brilliantly.

Oh yes - dont be afraid to take aggro on a couple of mobs, with the voidy. Send in the voidy on target1. let it get aggro then dot that target, then switch the voidy to the next mob and dot that and finish it off. A priest healing the voidy just makes the voidy amazing.