1) Developer is a base, or alkali, this means that a developer will increase the PH, until it's either neutral (7) or higher. An acid stop will keep an acid bleach an acid.
2) Acetic acid is what gives vinegar it's kick. The dilutions used in photography, typically 1% to 2%, is actually lower then that used for cooking use, typically 5% to 8%. This means if you run out of stop bath, go to the kitchen, grab the white vinegar from the cupboard and cut it 1+4 with water and you have a stop bath. If you have higher concentrations then for household use, it could be dangerous. Most acetic acid for stop bath is produced chemically these days, it's not the acid that could be dangerous, it's the impurities that could be in it, from the chemical process. Not much you can do about the smell. Another option would be to pick up a bottle of Ilford odourless stop bath, which is citric acid, mix according to the instructions and you should be pretty close. Mix according to need, in other words if you need 250ml of solution, mix that amount.