Check out Chin Chin at NYNY. It's a Chinese restaurant that offers most of their dishes with tofu instead of meat. When we go to Vegas we usually go there a couple of times. The food is very good.
When we go in December we're going to try out this place
which is at Caesars Palace.
A friend of ours went here
in Sedona and recommended it highly. He's not vegetarian, so you know it's got to be good.
And Happy Cow will give you lots more ideas.
Have a great trip!