When Vexing Devil enters the battlefield, its ability triggers and gives the opponent the choice of either allowing it to stay on the battlefield, or taking 4 damage so its controller sacrifices it. If you allow the opponent to make a decision on whether or not they take 4 damage, you cannot cast Fling “in response” to that choice before the creature is sacrificed. That’s because when a spell or ability starts resolving, you have to wait until all parts of the spell or ability are complete before you get priority again.

• If you cast Fling by sacrificing the Devil, it will be before the player makes a choice, and they won’t choose to take 4 from the ability because the Devil has already been sacrificed for Fling.
• If you cast Undying Evil on the Devil, it will be before the player chooses whether to take 4 damage, and then they’re probably not going to take 4 damage only to have the Devil come back as a 5/4.
• If you cast Cloudshift on the Devil, it will be before the first ability resolves, and the player will only have to worry about taking 4 damage for the new ability that triggers when the Devil returns from exile, and can gladly ignore the earlier ability on the stack by doing nothing as it resolves.

Today’s Rules Tip written by
Ronny Alvarado, Level 2 judge from Houston, TX

(It seems this post didn’t appear in the twitter feed on April 30, so I’m publishing it again)