Yeah, you definitely can't take fruit on the plane. I remember when I was going through security on my way out of Maui, they had a collection of apples and oranges, which apparently had been confiscated. I don't know about other types of food. I had coffee and sugar in my checked luggage when I left, but that's the only food I had with me.
As far as airports, I've gotten a vegetarian sandwich before (it had cheese on it, so wasn't vegan), but other than that I haven't noticed any vegan selections anywhere. When I flew to Hawaii a meal was included on the flight. I had pasta, which may have been vegan, but I think it probably had cheese on it.
I also agree that groceries can be expensive, but I don't recall them being outrageously so. If you go to Foodland, I think, you might be able to get a Maika'i card, which will give you discounts on some things, just like a Kroger card. I'm not sure about the requirements for getting one, but it couldn't hurt to ask. I think I just asked for one and don't recall ever filling anything out. Just pretend that you live there.
If you're looking for a health food store, I know there's one in Paia called Mana Foods, although, you're probably staying in the Kihei area, so that may be a little far away. It's worth going to Paia anyway, in my opinion. That was my favorite town and has all the best restaurants, again in my opinion. I wasn't veg*n when I lived there though, so I can't say how vegan friendly they are, but there was a vegetarian, possibly vegan, restaurant there called Fresh Mint. I think it was Vietnamese. Anyway, Paia's about four miles from Kahului on Hana Highway.