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Target creature gets -3/-3 until end of turn. You may tap any number of untapped Vampire creatures you control. If you do, put a +1/+1 counter on each of those Vampires.
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.
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.
You don't choose which untapped Vampire creatures you control to tap (if any) until Urge to Feed resolves.
