2010-03-01 : If the targeted creature is an illegal target by the time Urge to Feed resolves, the entire spell doesn't resolve. You can't tap any Vampires.

2010-03-01 : If you cast Urge to Feed targeting an untapped Vampire creature you control with toughness 3, its toughness will be reduced to 0 by the -3/-3 effect. However, state-based actions aren't checked until the spell has finished resolving. You may then tap that Vampire and put a +1/+1 counter on it, raising its toughness to 1. It will remain on the battlefield.

2010-03-01 : You don't choose which untapped Vampire creatures you control to tap (if any) until Urge to Feed resolves.