Do not think the damage would be given to the caster since vengeance does not count as anyone's damage. Unless it was just changed with the group veng update.
I know its crazy

But i know this because lvl 3 skillers often use it to complete quests and get drops from monsters. They would deal at least 1lp damage to the target then use either recoils/veng/poison to finish off the monster and ensure no extra xp is gained.
I know poison/recoils/veng apply like this. I am not sure if it applies when veng is cast on self, but I know it works for sure if it is cast on another player.
But group vengeance sounds so awesome! Ive already seen videos in which a group would fall in and cast it before a mage casts a multi spell. Instantly killing the mage. Very awesome stuff!