at mythic level 10, your damage becomes all holy damage and vital strike breaks, doing only a single regular-damage attack. just be aware when entering the final boss fight. i listed this bug on the vital strike ability page too. it works for companions still. i quit to desktop and reloaded two different saves to see if maybe it was a saving or loading error, but both had the same problem. i didn't notice it on the first save, but the second time around, i caught it.