I'm uncertain on Vengeance stacking (I assume the same as flowing thought items). As of quite some time ago, they stated in a patch message that all flowing thought items stack. I do believe the cap is FT 15. That means you can wear 15 flowing thought 1 items or 3 FT5 items, etc.
Alcho 57th Myrmidon on The 7th - 15AA and counting!
this is a very nice melee cloak, great stats and V3, it is a shame what you have to go through to get it but you can't enjoy a reward with out the effort to get it i guess.
Vengeance may well be +5atk per rank, but all melee get some bonus, seems to be around 1.4, which will make it 7 atk. All vengeance and other atk buffs stack up to the cap, which is 250 worn atk, i believe...