My wife and I have come to Maui almost every year for several decades. Being vegetarians (but not vegans), naturally the many seafood and steak restaurants don't interest us.
Good veggie food does, particularly if it is local and organic.
Since we always stay on Napili Bay, we're most familiar with vegetarian dining on the Lahaina side of Maui. Which, sadly, isn't as veggie-friendly as the funkier areas of the island such as Paia.
Here's some of our favorite places to eat vegetarian on Maui:
-- Whole Foods Market and Down to Earth Natural Foods, Kahalui. These are our first stops after we pick up our rental car at the airport. Whole Foods is new to Maui; Down to Earth is the long-time local natural food store. Each has a good selection of vegetarian take-out. Down to Earth is completely veggie with a more pleasing vibe. We buy a bunch of groceries at each before heading to our Napili condo
-- Mala Ocean Tavern, Lahaina. This usually is our first dinner eat-out spot. It's right on the ocean (but make a reservation if you want an outside table; Mala is busy around dinner time). We share a Farmer's Salad and the Hummus Quartet plate. The pita bread is excellent, along with the service. There aren't many other veggie options, but these two are worth a vegetarian's visit.
-- Lahaina Coolers, Lahaina. This restaurant and bar is just a few blocks from Front Street. It's open late, so we like to peruse the shops and art galleries early in the evening, then head to Lahaina Coolers for a meal. I love the Local Harvest Stirfry: green soba noodles, veggies, and fried tofu. My wife wasn't as wild about her Red Pepper Risotto; she liked the grilled veggie skewer and wanted more of it, with less risotto.
-- Hula Grill, Kaanapali Beach. One night during our stay we head to the Whaler's Village shopping center. After wandering around for a while the Hula Grill is our favorite dining choice. It's best to walk into the dining room, rather than the Barefoot Bar side of the restaurant. You can still eat outside by the beach, but you get a choice of three menus. We like to order a couple of veggie side dishes from the dining room menu, along with the $10 Tofu and Vegetable Stack. Healthy and relatively inexpensive dining in a great beachfront atmosphere.
-- Aina Gourmet Market, Lahaina. We've only been here once, as it recently opened. It's sort of hard to find, especially in the dark. The Market is mostly take-out, but the pleasant server/barista girl heated up some dishes for us in the microwave. We ate them at a small coffee bar, then took an oceanside stroll along the Honua Kai Resort grounds. The food is organic, local, and pretty spendy. Still cheaper, though, than eating at a regular restaurant.
-- Flatbread Company, Paia. There's quite a few good vegetarian eating options in funky Paia, on the other side of Maui from Lahaina. If you're in the mood for pizza, you can't go wrong at the Flatbread Company. I can't recall ever eating a better veggie pizza than the Vegan offering. We added some goat cheese and organic rosemary to it. My conclusion: every veggie pizza should have avocados and be cooked in a wood-fired oven.
-- Pita Paradise, Kihei. This is my favorite eating spot on the south side of Maui. If they had a Pita Paradise back in Salem, Oregon, I'd be ecstatic. What a great idea: to offer homemade tasting pita bread filled with all kinds of delectable options, some of which are vegetarian. This informal restaurant is next to a covered shopping bazaar, which adds to its "must eat here'ness" for us.
-- Thai Chef, Lahaina. Sometimes we stop by Thai Chef and get some takeout food for eating in our condo after putting in a calorie-depleting several hours of shopping on Front Street. I wouldn't say that this is the best Thai food I've ever tasted, but it's pretty darn good -- though I agree with some Trip Advisor reviews that the portions could be larger. Still, it's another good option for vegetarians, albeit with a shopping center atmosphere (which is why we usually get take-out and return to Napili Bay).