You donâ€™t choose which untapped Vampire creatures you control to tap (if any) until Urge to Feed resolves.
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 arenâ€™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.
If the targeted creature is an illegal target by the time Urge to Feed resolves, the entire spell doesnâ€™t resolve. You canâ€™t tap any Vampires.
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