If either creature is an illegal target as Vivien’s second ability tries to resolve, the creature you control won’t deal damage.
2018-07-13
Vivien’s last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t get +4/+4 or gain trample.
+2: Put two +1/+1 counters on up to one target creature.
−3: Target creature you control deals damage equal to its power to target creature you don't control.
−9: Creatures you control get +4/+4 and gain trample until end of turn.
