Combine the vinegar, sugar, salt, and 1 cup of water in a small saucepan over high heat and bring to a boil. Place the jalapeños in a heatproof bowl and pour the boiling pickling liquid over them. Set aside to cool to room temperature. Store the jalapeños in an airtight container, submerged in the liquid, in the refrigerator. Once fully chilled, they are ready to eat, and will last for months in the refrigerator.