Adds do have a large aggro range. Adds spawn based on Vicar's hitpoints (there will be 5) and they only spawn once. So if you gate or die before killing Vicar, when you re-engage no further adds will spawn until you reach the next hp trigger, though they will remain up until killed. The adds are mezzable. She is not invulnerable while adds are up as of this post.
The quest borked up when I killed the vicar first. She is off to the side a bit don't make my mistake and kill her before beating up the gnome. She didn't drop anything the divining stone you need and there was no chest up to search.
I was able to get her down while adds were mezzed, but then ended up with so many adds that even AE mezz was not enough. When you come back into zone and then get near her on road (big agro range) she and all the adds agro.