The vegetable hoagie is the Philadelphia hoagie with the cured meat taken out and nothing pretending to replace it. It is not a vegetable sandwich that happens to be on a hoagie roll; it is the standard hoagie build, provolone and the full dress, run without the capicola, salami, and ham that normally anchor it. That subtraction is the defining fact. The cured-meat trio is what supplies the fat, the salt, and the savory weight in an Italian hoagie, and removing it forces the cheese and the dressing system to carry a load they usually only assist with.
The craft is in making the dress do nearly all the work. Provolone becomes the single source of fat and salt, so it is laid on more generously and shingled the length of the roll rather than treated as an accent. The oil and oregano are no longer lubricating a stack of rich meats; they are the primary seasoning, and an underpoured vegetable hoagie tastes like exactly what it is, lettuce and bread. The raw onion and the hot or sweet peppers carry the acid and the sharp top note that the absent meat would otherwise balance. The real structural problem is moisture: with no fatty meat to buffer the bread, the tomato, lettuce, and oil sit directly against the crumb, so the Philadelphia roll's crust matters even more, and the tomato has to go in as part of the dressed structure rather than a wet afterthought or a foot of sandwich fails in the middle. Built with attention to the pour, the cheese and the dress make a balanced sandwich; built carelessly, the missing meat is all you taste.
The variations track what gets added back to fill the gap the meat left. A roasted-pepper-and-provolone build leans on a sweet, oily vegetable for the richness; a marinated-mushroom or eggplant build brings a meatier texture and savor; an avocado build borrows the fat from produce instead. The same long roll wears other cities' names as the hero, the grinder, and the wedge, with small dressing and bread differences locals defend. Each of those is a codified build with its own rules, and each deserves its own article rather than being crowded in here.