I'd keep the VW out at lower levels. You really don't want to use the imp when you are soloing until you get some more stam and damages gear and you get dark pact to suck the mana out of the guy.
Yah. Drain tanking is nice and all, but not really recommended sub level 40 (and definitely not sub level 20). You CAN juggle aggro with a succubus so neither of you takes overwhelming damage after you have Fel Concentration, but the build really comes into its own after Dark Pact.
(.. and continues to be the most obscenely effective grinding build ever all the way until 70. Speaking from experience, here. :P)
Also, I always open with Curse of Agony. It's damage increases with each tick until the heavist damage is at the end of the cycle, and I like to get the most out if it I can. Immolate is usually what I save for last, if I cast it on what I'm killing. I'm finding that, for now at least, if the stuff is my level or a little lower, it's usually wasting mana to add that on top of the other DoTs.
As affliction, the only thing I use the voidwalker for is killing unfearable, unslowable elite mobs for quests. 95% of the time I have my imp or felhunter out when solo... at 60+ with a lot of shadow damage gear, dark pact, and a passive imp mana battery I can keep effortlessly chain-killing an arbitrarily high [6+] number of mobs.
"The first rule of fel armor: you do NOT talk about fel armor." :p
-- Z.
Um, yeah. I was sub 62 when I read this and didn't understand.