The panino vegetariano is defined by a problem the meat versions never have to solve: with no cured salt and no rendered fat in the filling, the vegetables have to supply structure, savour, and a bind of their own, or the sandwich is a damp pile in soft bread. The element that makes it work is not any single vegetable but the dressing logic that holds them together, usually grilled or roasted vegetables bound by oil and something sharp, or a soft cheese acting as mortar. Aubergine and courgette charred until they collapse, sweet roasted peppers, a smear of stracchino or fresh ricotta, a few leaves with bite. The fat and the acid are not a finishing touch here; they are the part that turns separate vegetables into one thing the bread can hold.
The craft is in cooking the water out and putting the fat back. Grilled aubergine and courgette are taken far enough that they go soft and smoky rather than squeaky, then drained and dressed in oil with garlic and a little vinegar so they stop weeping and start tasting of more than themselves. Roasted peppers are peeled and dried for the same reason: a wet pepper soaks the crumb and tastes of nothing. The bind is deliberate, a film of soft cheese or a thread of good oil sealing the inner crumb so a juicy filling does not go to paste. Seasoning is heavier than it would be over meat, because vegetables need salt brought to them. The bread is plain and sturdy, a ciabatta or a country roll with enough chew to take an oily, acidic filling without surrendering. It is layered so the wettest element sits between two drier ones, and eaten before the oil migrates fully into the crust.
The variations are really separate sandwiches under one heading, and most deserve their own articles rather than being crowded in here. There is the grilled-vegetable build proper, the caprese of tomato, mozzarella, and basil, the one carried by a melting cheese rather than oil, and the regional vegetable spreads, a Sicilian caponata or a Ligurian pesto with green beans, each of which is a distinct construction with its own bind to strike.