This is how Vengeance stack works
I think I have worked out this Vengeance stack and how it works.
The Vengeance stack has a list of different Weapons Dmg melee bounus, and you can get various combinations of them depending on how you feat and AA's
15%, 15%, 5%, 3%, 6%, 9%, 12%.
so a Vengeance stack is something that all guards have innately. Though it only procs when your in a group and a group member is getting melee hit. (Thanks to Stian for enlightening me to this fact) This will be 15% when stack is x 10.
Your can max out an AA so once you get to stack x 6 you get 1% bonus up to a 5% bonus at stack x 10.
In tempest tree of feats you have Hateful vengeance and it gives 3% for every point. and most everyone who feats this will be after storm strike so it will be maxed out with 5 points which will give a bonus 15%.
So max I think you can get is 35%, this is from the base 15% that the stack has plus the extra 5% from the AA maxed and Hateful vengeance will give you an extra 15% if maxed.
So there you have it. I hope this makes sense, it does to me now.