WW can be used for just about any dietary restrictions or food choices as it really just calculates intake. And as long as you food is within the intake limits it can work.
I've known a number of vegetarians who've followed the plan, and I occasionally see a vegan or two mention that they're using WW on 3FC. If you're interested in changing gears and moving over to Vegan then there's also a Veg/Vegan section on 3FC where people might have some nutritional advice.
I do know that there are plenty of non-animal foods that still contain protein like beans and other legumes. Some grains, corn, peas. And then, all manner of nuts. Oh. And Tofu.