Now that Agli Alboretti has been taken over by Paolo of Vini da Gigio, and is run by his son, Nicolo, it is one of our favorite places in Venice. They always have a vegetarian option in both primi and secondi, and they are delicious. Colleen, are you a no living critter vegetarian or just no meat? If you eat fish at all I would put, al Paradiso, Carampane and Vini da Gigio at the head of my list along with Agli Alboretti. All three are favorites and never disappoint, but I am not sure how many vegetarian options they offer. Ristorante Riviera used to have at least one delicious vegetarian dish on every menu, but we have nto been back since they changed hands - too heart breaking not to see Monica in the kitchen.
On a more modest note, we always enjoy the Birarria in Campo San Polo - they have fantastic pizzas, and some vegetarian dishes on their regular menu. Be forewarned - they will happily serve the same table pizza and offerings from the regular menu, but your dishes will not arrive together. They have greatly improved their wine list too.
I could easily make a lunch out of the fresh fruit - you can buy filled containers or choose your own selection at Dersut alongside of the Frari. Poste Vecie is another favorite, but they seem to be under new management, and sadly, I have read some negative comments about them. When we were there in April, it was still very good though we only saw one or two familiar faces, but April is now more nearly nine months in the past.
My problem with many places is too many dairy filled dishes - especially pastas - so I understand that it can be difficult to rule out categories of food and still find the meal of your choice at the restaurant everyone else wants to visit. Giordano at al Paradiso and Franco at Carampane will probably go out of their way to fill your desire for something vegetarian especially if you give them a little notice when you make your reservation. Mamma mia do I wish we were gong this spring! Ruth