Szob, one of the problems VG tanks face is that they don't have any true spike reducing cooldowns like assassins.
NIM mode fights usually have alot of damage that cant be shielded/defended ,so VGs get pounded by internal/elemental damage, due to no spike eating abilites and few defensives.
An assassin can shroud these big spike hits(taking 0 damage) and still run higher endurance without huge negatives
Reven uses mostly Internal damage for most of the fight if I remember right, and your healers may have to be better at
healing him.
Dps needs to not take unnecessary damage(not standing in stupid),as this will make it much harder to heal the tank.