On a recent visit, we had DW's aunt and uncle come over from Clearwater to meet us. She's not a true vegetarian - she can eat a little white meat, but her body doesn't produce the enzyme needed to digest most any other meats ... (sorry, TMI).
Anyway, our server apparently saw her distress, and without asking, brought her a nice vegetable/Asian noodle dish - sort of a lo mein. She loved it.
I think simply asking would do the trick, but perhaps for peace of mind, a phone call ahead would help.