Wumb

Not really sure about that.. I mean, think about it. Having it activated when party members take damage would activate it the whole time since your party is attacked in every fight - there's simply no situation where you insta-kill a group of mobs or a boss, so that equals into party members taking damage everytime. That would be a free buff, like a constant passive effect, and that would ruin the talent (making it something like the same as "increase haste by 5%", you know?). :/
And about making it longer... not sure about that either. 30 seconds would be way too long, also would that result in healers pulling the aggro from mobs/bosses just to activate the effect every 30 seconds; that would not only annoy you (probably), but also the tank since he has to regain all the aggro that's lost by that. Holy Vengeance is a "rescue tool" in my eyes: You take damage, you get a benefit to quickly heal yourself without spending too much time on it so you can heal others again as soon as possible. 5 seconds is enough to heal yourself up to 100%, especially with a haste bonus.
Did you ever wiped with your whole group and thought "Ah, if Holy Vengeance had this and this effect, everything would've worked so much better.."? I don't think so. .-.

i mean 5 second serenity with 30% dmg reduc..whos idea was that.

Use an autoattack and the reduction is gone. Serenity just reduces the next incoming damage...