Yes indeed! In fact, Phase I is definitely vegetarian! You could probably modify it in the subsequent phases to add some protein from dairy or tofu or something like that.
I agree -- Phase I is a wonderful for vegetarian. Can anyone share some of their Phase II vegetarian meal plans? I have been using veggie burgers, adding the 1/2 avocado and salad. Beans and greens are still a staple but now I add tvp or other crumbles to my veggie chilis.