Cut the jalapeños into thin slices. If you like your peppers hot, cut all the way up to the stem; for a milder batch, stop a 1⁄2 inch before. Combine the vinegar, water, salt, and sugar in a saucepan and heat just enough so that the salt and sugar dissolve. Allow the liquid to cool briefly. Place the jalapeños in a sterilized jar or small mixing bowl. Pour the liquid over them, then cover, letting them soak for at least 10 minutes before using. Will keep for a week covered in the refrigerator.