If you are spending time vamping, you are _not_ casting scourge, poison, curse, weaken, blind, etc. Playing a maledictive class like a necro, shaman, or an AP (to a lesser degree) is like tying an elaborate knot - you have to lay out your rope before you cinch it tight with direct damage.
Also, if it gets to the point where you have to use a vamp touch to stay alive...you should probably be running. Considering the damage can hit pretty low if your opponent has decent saves, it's unreliable. Still, I did make it work a few times in PK, but those are generally fights where I see my army doing huge damage and I think right away that I better hit them hard, because as soon as they flee it'll be word/teleport. Or, I have all the aforementioned spells on them.
And ONE MORE thing. If a necromancer has such an upper hand that he feels he can vamp his victim, he probably won't want to simply because of the 2-round casting lag...generally, people will be trying to run from you. I used energy drain, ghoulish numbing, and chill touch a lot as my combat-initiating spells.
Hope that clarifies things.