I don’t think doing the PCT as a vegetarian is hard. I hiked 900 miles of the PCT in '04 and mostly resupplied in stores. But, it depends how strict you are - do you care that Liptons has a small amount of animal product? Do you eat rennet? etc. You do see a lot of vegetarians that will eat meat on the trail because veg food is a bit limited and your body is craving calories, but you certainly don’t have to.
I was happy eating mashed potatoes, Liptons and Ramen(no flavor packet) and usually mixed in a dehydrated soup mix for flavor. I did send a couple of homemade dehydrated meals to myself because I missed good food and veggies. Lunch was a bagel sandwich with cheese and veggie meat. A lot of stores (even in small communities) carry vegetarian lunch “meats” - they usually don’t have the goodness that is Tofurky, but they will have some of the more mainstream brands. Depending on how strict you are, there is usually plenty of restaurant food for a vegetarian to eat, too.
Doing the trail vegan is a different matter. I know there have been some vegans on the AT, but I have only heard of one on the PCT (I’m sure there have been more, but one is all I have heard of). I’m sure it can be done, but it would take a lot of planning, mail drops and possible deprivation at restaurants (not too many vegan options, unless you could iceberg lettuce and french fries).
Good luck!
Jenny