Explicitly vegetarian food is not at all in the culture of this tradition-obsessed island. This is a pity, since all the delicious ingredients can be found, including the typical Italian creativity. La Cucina dei Colori puts them together in simple dishes, some of which are surprising and delicious. They offer cooking courses as well (almost surely not in English). I recommend ordering a mixed plate with 3,4 or 5 different things. When I was there, everything was vegan or vegetarian.
I also recommend to try the desserts. Think "nerdy creation, almost healthy" rather than "sugar flash" when you order the dark chocolate mousse.
Cons: Uncozy hall atmosphere, No good website with opening times