I would think for a Vegetarian BBQ, and I honestly don't know why anyone would request a vagetarian BBQ from a BBQ caterer, I would not try and copy meat BBQ. Veggie burgers are rarely any good on open flame (and yes, I have tried all of the commercial brands and about a dozen home made ones). They need the oil from a griddle to sear.
I would look at grilling vegetables, doing hot casserole type dishes that layer veggies and sauce, things like that, which can be cooked on a smoker or grill and will take on some smoke flavor. I did try some vegetarian ribs once, that were quite good, but, were all about the sauce. BBQ sauce is generally vegetarian, so there is that. Hehe, pulled spaghetti squash sandwiches...there is an idea.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less