Unless you're fighting a felar, invoker, cloud, etc - I'd opt for chill touch. It's a 1 round spell, so you can get 2 of them in, which will do 2 things:
1) Usually 2 chill touches > 1 vamp touch for damage
2) Gives you more flexibility. You can do 1 chill touch and a crimson, a blind and a chill touch, etc.

I'd save vampirc touch for mobs, mostly - unless you want to extend a fight with an opponent you're not worried about fleeing