The Pan Bagnat Végétarien is the reading defined by a single substitution: the fish comes out and nothing fishy replaces it. No tuna, no anchovy. What remains is the rest of the Niçois set, hard-boiled egg, tomato, raw onion, small black olives, sometimes a leaf of basil, sometimes a stripe of raw red pepper, packed into a split round country loaf, dressed with olive oil and a touch of red-wine vinegar, weighted, and left to rest. The egg stays, so this is vegetarian rather than vegan, and the structure is otherwise unchanged. The defining question of the version is what carries the savour once the two saltiest, most marine ingredients are gone.
The answer is the soak doing the work the fish used to do. Without anchovy supplying cured salt and tuna supplying oil and body, the olive oil and vinegar have to season the crumb on their own, the tomato has to be ripe enough to carry the sandwich, and the olives and onion have to stand in for the depth the fish gave. This is why the vegetarian reading lives on the Côte d'Azur mostly in high summer, when the tomatoes are loud enough to anchor it; out of season, with thin tomato and no fish, the build goes flat. The bread requirement does not relax: it is still the sturdy, faintly tart Niçois round, still soaked rather than crisp, still improving over its first hours under weight. Capers are the most common addition, brining back some of the sharp salt the anchovy used to provide.
Variations work to rebuild the savoury depth the fish carried. Capers are the standard move; some hands add fresh mozzarella or a few strips of grilled pepper for body; a more assertive olive does the same job from the cure side. None of these reintroduce fish, which is the line that defines the version. The rest of the inherited orthodoxy holds: no cooked vegetables, no mayonnaise, the bread soaked rather than crisp, patience as the technique. For the full account of the soak, the bread, and the Niçois rules this version inherits, see Pan Bagnat.