The place is quite popular among the tourists in general, not only vegetarians, due to its democratic prices and the fact that it's located near Xom Moi market, close to tourist district. As it turned out, some tourists don't even fully understand what they order, and imagine omelet and "unsalted fish" among the variety of soy dishes. :)
The food is rather tasty but it's the typical Vietnamese fast-food so don't expect much diversity. I would recommend you to try "bánh canh". Here it's made from the thick wheat noodles and served with the delicious fried tofu - quite original. By the way, Bồ Đề is a typical "chay" (vegetarian) place, and you can't say that it's fully vegan. At least, if you want you can easily order cow milk as a drink. As for the other dishes, it's most likely that they are vegan, but nobody will guarantee you that, because it's unlikely that the concept of "veganism" is familiar to Vietnamese buddhists.
Bồ Đề is the dirtiest place among all the vegetarian places in Nha Trang, probably because of its large size. This is the main drawback. It isn't extraordinary event really to see a cockroach running along the wall or on your table. And if you're kind of squeamish person and you're afraid of the mere thought of finding a little bug or a black hair in your dish, you should never go to Cơm Chay Bồ Đề. :)
Cons: Dirty, Little variety in dishes