If you remember from last week, Vengeance is being changed in patch 5.4. If like me you've been curious about how it will specifically be changing, Lore over on the official forums has our first explanation of the changes and how they'll shake out. Here's the breakdown.
So if tanking a small group or adds that accompany a boss mob, this change will have little effect - it's when tanking huge packs that the vengeance decay will be most noticeable. The calculation based on most damage seems interesting to me - hopefully the calculations won't be such that vengeance fails to stack properly.