This is one of those "standard" dishes you'll find in almost every Thai restaurant. When ordering ala-carte dishes, Thais will normally include one or two vegetable dishes, and this mixed vegetable recipe is the easiest choice; one all guests can agree on.
You can, of course, vary this dish to use whatever you have on hand. It's one of those recipes that you rarely make exactly the same way twice. To make it truly vegetarian, use mushroom sauce in place of oyster sauce.