Whilst Hoi An felt a bit like a Disneyfied version of Vietnam to us, there is a restaurant on the waterfront which did the best vegetarian food we ate in the whole of VN.
Cafe des Amis serves a set meal each evening (I forget how many courses). The only choice is seafood or veggie and it is utterly delicious food. If you are veggie, you'll be sick of spring rolls and stir-fried greens by the time you get to Hoi An, and Cafe des Amis will provide some very welcome and tasty variety.
Updates
In Dec.'07 this bistro was magic - we ate at Brothers for four times the price, and while the service and food there was better, marginally in my opinion, give me the menu de jour here. Bit grimy but it works! Went back three days straight. Bordeaux and atmosphere make it, but be aware you choose one of three menus - meat, fish or vegetarian. Each is four courses plus for under AUS$20 per head. Looking out over the passing river front with pedestrians (where Kim sits with a wine and his dog 'Chocolat') on a balmy night is what makes this a memorable experience. Best value by far!
Is this your business?