How heavy are you? How tall are you?
My sister is a non meat eater, and I won't recommend the diet to her even so, if you're a true vegetarian, while I wouldn't say it's "unsafe" (as I know many veggies who live on pizza and other packaged rubbish), but you'll get bored very very quickly in my opinion.
Bren is vegetarian, and stuck at it admirably for quite a while before (non related) health issues got in the way. Search back a few pages in the diary section and you might find her diary from last year.
I can't recall now but not only yes can you eat tofu and quorn, but the quantities of oatbran are higher for veggies. Again, you'd need to do your own research for this as I can't remember (it's not in the book - just advice he's given on the official website).