I just killed Pit Lord Vilemus on two of my characters, and neither had the bonus roll option pop up. Also no one else in my groups was able to bonus roll. Is this happening to all of you?
We've had some reports of this happening and it's currently being looked into.
As others have mentioned, a workaround right now is to re-enter the Greater Invasion Point and kill the boss again and you should then receive the Bonus Roll prompt.