The tramezzino vegetariano has to solve a problem the meat and fish builds never face, which is how to make a soft pale triangle feel substantial with nothing cured or oily at its centre. The answer is a layered build of vegetables held by a fat bind: typically grilled or marinated vegetables, lettuce, tomato, sometimes egg or a soft cheese, stacked between two slices of crustless white pancarrè. No single element carries the sandwich the way tuna or salmon does in its own version; instead the parts are arranged so colour, moisture, and crunch overlap into something that eats as a whole. The bread gives the soft frame and a mild backdrop. The mayonnaise or soft cheese binds the loose stack and seals the crumb. The vegetables give the body, the freshness, and the bite. Pull the bind and the stack falls out the open side; pull the vegetables and there is nothing to the sandwich at all. The parts are engineered to need each other because no one of them can stand alone here.
A good one lives and dies on moisture control, since vegetables are mostly water. The loaf is soft white pancarrè, fresh that day, soft to a thumb, all crust trimmed so only the tender crumb is used. Wet vegetables are managed before they meet bread: tomato is seeded and blotted, grilled vegetables are drained of their marinade, lettuce goes in dry and whole-leafed for lift, and anything weeping is patted down so the juice stays off the crumb. The bind, mayonnaise or a soft fresh cheese, does double duty, gluing the loose layers into one body and laying a sealing film on the inner faces of the bread so residual moisture meets fat rather than bare crumb. The stack is kept even and mounded toward the centre so the triangle domes, fullest in the middle and thin at the cut, rather than thick at one end and bare at the other. Seasoning is light, salt and pepper and maybe a thread of lemon, because the vegetables already bring their own range. A sloppy build piles wet unblotted vegetables on dry bread and serves a grey collapsing corner; a careful one dries every layer, seals the bread, centres the dome, and cuts a diagonal that holds its bright cross section.
The close cousins each deserve their own article rather than being crowded in here. Lean the build entirely on grilled peppers, courgette, and aubergine and it becomes the grilled-vegetable tramezzino, a smokier single-minded thing. Centre it on tomato and mozzarella and it tips into caprese territory. Add egg and it borders the egg builds. Strip it to plain tomato and lettuce and the math simplifies again. The vegetariano is the open layered baseline of the meatless side of the family, and each tighter version is best read on its own terms first.