"vegetable diet" is correct and usable in written English.
You could use it in a discussion about healthy eating or nutrition, for example. Example sentence: I'm trying to lose weight, so I'm going to start a vegetable diet.
Paul reported that "for a long time he [McVeigh] was on a vegetable diet that comes in a special tray.
So I din'd upon Cod very heartily and continu'd to eat with other People, returning only now & then occasionally to a vegetable Diet.
Hyde would suggest to Bernard Shaw if he doesn't want to be left behind, he had better exchange his vegetable diet for good red meat and write virile plays for change.
The roast beef of Olde England was character-building food, stout fare for stouthearted men, while it was widely presumed that a vegetable diet made men weak, timorous, and effeminate.
While those most affected are "women of a delicate habit," (a milk and vegetable diet and "cold bathing" are prescribed), Dr. Buchan does offer this warning about mass hysteria: "When hysteric fits are accompanied by sympathy [in its old sense meaning shared feelings by many], they may be cured by exciting the opposite passion".
Such is the rainbow of humanity that descends on Worthy Farm each year that, as legend has it, if you've not spent at least a few lost hours dissecting the pros and cons of a raw vegetable diet with a complete stranger in the Green Fields then, frankly, there are less muddy ways to pass a weekend.
Similarly, in 1796, the American Philosophical Society announced rewards for "the best performances, inventions, or improvements" in ships' pumps, calculating longitude by lunar distance, stoves, preventing decay in peach trees, studying native American vegetable diets, and street lighting.
Low-fat diets have appeared superior to high vegetable diets, but not always.
Similarly, antioxidants in foods, especially fruit and vegetable diets, may counter the oxidative damage in tissues.
Flavonoids, a class of natural polyphenolic compounds, are widely found in abundance in fruits, vegetables, and other vegetable diets.
Fruit and vegetable diets contain large amounts of antioxidants, which slow the development of cardiovascular disease (Gaziano 2004).
