At 40 you get Dark Pact. This is where the rift between Drain Hunters (Mike) and Drain Tankers (~50% of us) happens.
For the lot of us that are Drain Tankers,40 is a good time to stop using VW... permanently (the exception of course being farming with Seed of Corruption, which since 3.0 now puts the VW in an even less useful place.)
Unless you are a good bit into Demonology, which most of us don't use for leveling at least until Felguard is viable, the VW begins slacking heavily in the Agro department. Ontop of that, his mana regen is so bad that in 'intesive grinding' you are required to resummon him so he can start with fresh mana ever 4 or 5 fights (which litterally is 2-3 minutes.)
The Imp has impressive Spirit, whose mana regeneration is only amplified by 3.0 to be that to the tune of ~300mana per tick(5 seconds) at level 40 (900 at 70). Thats much more than even a Arcane Mage or Disc Priest can pull off... As deep affliction that's almost more than you can spend! Keep him on Passive, Phased out. He wont enter combat and wont cast, meaning he'll be getting full Mana regen permanently. Dark pact for FREE MANA!
Using yourself as tank, or juggling monsters with fears, Drain Tanking becomes a very viable and fun option. Start with 1 monster at a time, Dot him up (before he gets to you) and then drain when applicable. Past that, you'll find that your Drains are actually overkilling (and therefor overusing your mana.) Your choice then is to either cut down on DoTing (the easy way) or DoT, and then pull another monsters (the faster, harder way.)
You can learn to get 6-8 Monsters on you at a time (in your prime around level 50.) Later in game, this becomes hard because you'll kill things too fast.
Using Improved Howl of Terror is a good way to collect 4 monsters and then take the damage off yourself.
Dont forget to Drain when you need, but if you're keeping all your targets Siphon Life'd, you'll be getting good hp back, especially when you get Fel Armor.
Your Drain Hunting (mike's) option is to scale back your damage. Yes, kill slower. But hey, it works too if you want a more controllable play.