Void is pointless if you have Fel Concentration. Go with Imp, Sucubus or Fel Hunter depending on what you want. Sucubus is the best pure DPS so that would be my recommendation.
Void can't hold aggro for shit anyways unless you majorly hold back (I don't remember hearing about them buffing him.
Apply dots (Immo, Corruption, CoA), drain life until dead, life tap, repeat. Once you get Dark Pact use that instead of Life Tap with a Sucubus with all skills turned off. The Siphon Life change is going to be a huge buff to leveling next patch since Siphon Life isn't generally worth casting as leveling (it's only good once you get some spellpower) but you always cast Corruption. Don't bother with Drain Mana, it's fucking worthless for PVE.
You should be able to manage your health and mana by doing that so that you can get away with just a bandage every few minutes. Once you get Dark Pact you should have zero downtime.