The salt and vinegar crisp sandwich is the sharp end of the crisp-in-bread tradition, and the vinegar is the reason this particular packet is the one reached for. The crunch is the structure of any crisp sandwich, but the choice of salt and vinegar is a flavour decision: an acetic, mouth-puckering tang dusted onto the crisp that cuts straight through soft white bread and salted butter in a way no other crisp does. There is no vinegar bottle involved and nothing wet added. The sourness is entirely the seasoning on the crisp, and the build is a packet of crisps, soft buttered bread, and the bread pressed down so the crisps shatter. The pleasure is a violent textural contrast carrying a sharp note where the cheese-and-onion version carries a savoury one.
The craft, such as it is, is the press and the clock, because the acid and the crunch are both on a fast timer. The crisps go in dry and are eaten soon, since the moment the butter's fat and the bread's moisture reach them they soften and the tang dulls into a flat sourness with none of the snap that was the point. Butter spread thick and to the edges is structural rather than incidental: it is the only binder gluing the loose crisps to the slice so the sandwich holds for a few bites, and its fat also blunts the vinegar just enough that the build reads as sharp rather than harsh. The bread is the softest, plainest white available, because the contrast only works if it offers no resistance of its own, and the firm push down crushes the crisps into a single brittle sheet so the sandwich bites cleanly instead of collapsing into a bag of fragments. The vinegar's job is to keep a starch-on-starch sandwich from reading as one bland note; the salt and the bread are what it is set against.
The variations are simply the crisp aisle, and they are a national in-joke with real internal logic. Cheese and onion swaps the sharp acid for a savoury dairy-and-allium dusting; ready salted strips it to pure crunch and salt; prawn cocktail brings a sweet-tangy note; the strongly seasoned and the branded shapes change the flavour while keeping the texture argument identical. The chip butty is the same starch-on-starch instinct made hot and soft instead of cold and crisp. Each deserves its own article rather than being crowded in here.