I was doing Nightmare Eternity Vault yesterday for the achievements that i was missing, and it turns out that 8man Nightmare EV grants you the wrong achievements.
Instead of obtaining the 8man Nightmare Avhievement for The Ancient Pylons and the Infernal Council, you are granted the 16man Story mode achievements.
Here are some screenshots:
http://i41.tinypic.com/6f353a.jpg
http://i42.tinypic.com/2q8y9mb.jpg