We were just visiting Zion in the off-season, and Oscar's was the only restaurant on the Happy Cow list that was actually open, so we decided to give it a shot. It seemed promising as the person who seated us told us she is vegan and she showed us how dishes that can be made vegan are designated on the menu. So we ordered a vegan enchilada and a vegan burger. After only a few bits of the enchilada, my friend had the unfortunate experience of biting into a chunk of chicken. While she ran off to gag and wash her mouth out, I picked through the dish and found another small piece of chicken. I called our server over and showed him. He took the dish back and we told him we didn't want it remade since we'd lost our appetite by then. He was super apologetic and gave us the "local" discount for our burger (even though we were there with a local friend who was eating the burger, but the gesture was nice, I guess). So be warned: it seems there are problems with cross-contamination in the kitchen!
Cons: We got meat in our food!