Kirsten, it seems that Luna is a "grass eater", like Peppito. When he goes outside, he can't help, just run and eat as much grass as possible (which usually shortens his outside walks very quickly!) and then he vomits, of course. Most of the time there is no hair ball. I think he is grass-addicted. Before we had a garden I was growing grass for him in the appartment, I could not give him before it was fully grown, otherwise within one hour there was nothing left! And when I gave him, he was pulling all of the grass out of the pot and eating, eating...
I am not sure it is a real need of grass in this case, he really looks mad when he smells grass...