There's an online Nigel Haworth recipe that calls for treacle vinegar, with no explanation of what it is, or how to get hold of it. Usually, treacle and vinegar would be used in addition to other ingredients in, for example, a barbeque sauce or marinade. In cooking, the vinegar will evaporate. Treacle and vinegar with breads sounds a bit odd. But you could make it yourself by adapting this recipe...
Common Molasses Vinegar
Mix together a gallon of West India molasses, and four gallons of lukewarm water. Pour it into a clean five-gallon cask, and place it in the chimney-corner; standing the cask on end, and leaving the bung out. To give it, occasionally, some additional heat, set the cask in the mouth of the oven on baking-days, after the bread is drawn, and let it remain while the oven continues warm. In three months it will be excellent and wholesome vinegar, at a very trifling cost, - only that of the gallon of molasses. When the liquid is sufficiently acid, stop the bung-hole closely, and remove the cask to a cool place. In summer, you may make this vinegar by letting the cask stand three or four months exposed to the hot sun; taking care to cover the bung-hole in damp or rainy weather.