Valentin, Dean of the Vein

If you gain life at the same time a creature you control is dealt lethal damage, that creature dies before Lisette’s ability can put a +1/+1 counter on it.

If a nontoken creature an opponent controls would die, and both Valentin and another effect are trying to exile it instead, that opponent chooses which effect to apply. If Valentin’s effect ends up not applying because of this choice, the triggered ability that allows you to pay {2} won’t trigger, even if the creature card ends up in exile.

If you gain an amount of life “for each” of something, that life is gained as one event and the ability will trigger only once.