Vindiami the Ancient will be non-attackable until you have killed four limestone watchers. These limestone watchers can be found inside alcoves around the zone, at:
( -219, 0, -94 ) /waypoint -219, 0, -94
( -200, 0, -61 ) /waypoint -200, 0, -61
( -161, 0, -93 ) /waypoint -161, 0, -93
( -146, 0, -145 ) /waypoint -146, 0, -145(Note: Recommended to keep this one alive until you have cleared the room where Vindiami is)
Each of these watchers will also be non-attackable until you have killed four brittle bones, each spawned by examining the four skeletons, two on each side of the watcher. Once all the brittle bones are killed, the limestone watcher will aggro. Repeat this for all four watchers, and Vindiami the Ancient will be attackable.
Like most former epics he hits harder than your average heroic, but still isn't too difficult to merc-solo.