I had thought that the adjective (h)olitorius, from the noun (h)olus, might work, since the meaning is 'of or concerned with vegetables.' But after looking at the attestations in the dictionaries and going through the search results on PHI, it doesn't appear that it was ever (at least in the extant literature, of course) used to describe food. It's mostly used as part of the name of the Forum Holitorium.
My new suggestion is hortulanus, 'of or belonging to a garden.' This is most commonly used, in its masculine forms, as a word for 'gardener'; but it also shows up in the name of a dish in Apicius (De re coquinaria 8.7.14):
Porcellum hortulanum: porcellus hortulanus exossatur per gulam in modum utris. mittitur in eo pullus isiciatus particulatim concisus, turdi, ficetulae, isicia de pulpa sua, lucanicae, dactili exossati, fabriles bulbi, cochleae exemptae, malvae, betae, porri, apium, cauliculi elixi, coriandrum, piper integrum, nuclei, ova XV superinfunduntur, liquamen piperatum [ova mittantur tria]. et consuitur et praeduratur. in furno assatur. deinde a dorso scinditur, et iure hoc perfunditur. piper teritur, ruta, liquamen, passum, mel, oleum modicum. cum bullierit, amulum mittitur.
Pig à la Jardinière
The pig is boned through the throat and filled with quenelles of chicken forcemeat, finely cut roast thrushes, fig-peckers, little sausage cakes, made of the pig's meat, Lucanian sausage, stoned dates, edible bulbs [glazed onions] snails taken out of the shell and poached mallows, leeks, beets, celery, cooked sprouts, coriander, whole pepper, nuts, 15 eggs poured over, broth, which is spiced with pepper, and diluted with 3 eggs; thereupon sew it tight, stiffen, and roast in the oven. When done, open the back of the pig and pour over the following sauce: crushed pepper, rue, broth, raisin wine, honey and a little oil, which when boiling is tied with roux.
(Translation from LacusCurtius)
Obviously, this dish is hardly vegetarian; however, in modern Italian cookery, dishes that are, if not totally vegetarian, at least very decidedly vegetable-based are sometimes referred to by using the descendent of this word – e.g., penne ortolano or rigatoni all'ortolana.