the vengeance change is a slight nerf in some ways and a buff in others its a nerf because in theory a tank soaking a massive hit could avoid attacks often enough after it that the vengeance from that massive attack would last indeffinately (this would take a lot of luck or a skilled druid + a fair bit of luck to happen though) and making it stack of avoided attacks isnt the buff people think it basically just means tanks not getting screwed threat wise if they get lucky and avoid half a dozen attacks right of the bat
there will be very few circumstances where this is actually a DPS increase for tanks