I'm visiting Venice in May with a vegetarian friend. (I eat seafood but not meat).
My friend does not eat meat or fish (i.e she is a 'proper' veggie - she also avoids eggs but dairy is OK).
I've visited Venice several times before and know some great seafood places - but I don't know of anywhere that either does both great seafood AND veggie, or specializes in veggie food only.
We are happy to go well off the beaten track - in fact we'd prefer to.