The vegetrian cuisine in Vietnam often calls their dishes like the ones containing meat, e.g. Pho Bo. But in restaurants with 'chay'(vegetarian) sign in front, those dishes are served with tofu instead of meat. Nearly every soup, sandwich and streetfood has its vegetarian correspondent.
Sometimes you can also see notations like 'pho chay', 'banh mi chay' (vegetarian sanwich) or 'com chay' (vegetarian rice).
The vegetarian food in comparison the normal dishes are almost always cheaper, often it's half the normal price. Chay restaurants are mostly frequented by religious viets and are rarely found in touristic areas.