The 'vanguard class' was a bit of false information. As others have stated, Bungie did mention 3 classes was not something set in stone, but currently that's all there is. The 3 classes they have covers the achetypes really well, they COULD add in a healer class or pet/turret class but I doubt it with the way they're focusing the game on action and what not, simply because those 2 archetypes are pretty passive and not directly combat oriented.