Cast Sequence questionFollow

Are you drain-tanking or drain hunting? Either way, I'd leave out Curse of Shadows and substitute Curse of Agony for soloing. Curse of Shadow only really is worthwhile in groups when you have another source of shadow damage (aka another warlock or a shadow priest), otherwise it's better to use CoA.

If you're not drain-tanking, I'd leave out Immolate, especially right off the bat, as that tends to make it hard for the voidwalker to pull aggro off you. For drain-hunting, my sequence (at least for an Aff build) is: send in VW, wait for the first Torment (or whatever its taunt is; I haven't used VW since I switched to Felguard), then cast UA, CoA, Corruption, Siphon Life, Life Tap the mana back, and Drain Life until full. If the mob's not dead by the time DL's done, wand the remainder down, or just wait out the DoTs. If the mob's only got a small fraction of life left, it isn't worth taking the time and mana to cast SB.

You should also consider trading Malediction for Improved Howl of Terror. Malediction is only worthwhile in raids, and then only for a single warlock. IHoT gives you a second insta-Fear (along with Deathcoil), which can be lifesaving when you get unexpected adds. You should also probably drop the single Shadow Embrace point, as since 2.3 SE doesn't add to Soul Siphon's effect.