I would actually use the imp for mana battery and go all the way to get Haunt. The reasoning is this: your main concern while leveling is not survivability. If you're careful, you shouldn't get more than 3 adds at a time, or even 2 if you really know how to pull. Your main limiting factor is your mana supply, which can be replenished through dark pact and life tap. If you do this, spec balance for druid just for added dps.

Alternatively, you can keep using VW, get soul link, and use him as your tank. For mana supply, life tap all you want, and druid keeps healing. Don't bother to heal VW, he can take beatings really well. Just heal the lock to keep mana supply up. That way, you're an unstoppable killing machine. For this approach, you can spec resto or balance doesn't matter. Even if you spec balance, you can still cast HOT once every 5 mobs to replenish warlock's mana and imo it should be sufficient.